GEOGEAPHICAL DISTRIBUTION OF THE NAIADES

317 though slightly pearly, i8 dull, and their beaks are plicately sculptured

II. GEOGEAPHICAL DISTRIBUTION OF THE NAIADES

In

mapping

out the generaldistributionof the]S^aiades, althoughthey are all contiuedto the fresh waters of the globe, itwill be found that theyfall into provinces something in the

same way

as do the other

members

oftheanimalkingdom. Sonearly do these areas coincide in a

number

ofcases withthoseof generally recognized regions ofanimal

life, thatin severalinstancesI have applied the

same names

tothem.

To

a considerable extent, as

would

be expected, these divisions of

Naiad

life are

bounded by

the sea,

by

deserts,

and

mountain chains which actas watershedsfordifferentriver systems.

Yet

noneof these inallcases effectually restrictthe distribution of thefresh-water mus-

sels;

and

it is true that in several instances the borders of a xvTaiad region are not

marked by any

tangiblenatural barriers.

ThePalearetieEegion.

—

^This,thelargestregionofXaiadlife,includes ina general

way

thewholeofEurope, Africa (excepting theNile),north of the Desertof Sahara; allofAsia north of the Stanovoi

and

Altai Mountains, including, probably, the greater part of Afghanistan

and

Beloochistan, Persia,Arabia,

and

Asia Minor;

and

allofNorth

America

thatisdrainedinto thePacific. This vast ^region, covering anarea of perhaps 10,000,000 square miles, is inhabited

by

asingle and remark- ably

homogeneous Naiad

fauna.

One

si)ecies characteristic of this province, Uniomargaritiferus,Linnaius,is foundin allparts of

Europe

except the region along the Mediterranean; also throughout Siberia;

in northern Japan, wliich stands on the border between this

and

the Oriental region, and in that i);irt of this i)rovince in North

America

328 THE PEARLY FRESH-WATER MUSSELS—

SIMPSON. ^vol.xvm.

lying northof about 40°; occupjang in all an area in the palearctic region of something like 9,000,000 square miles.

The Amoor

River, which takes its rise in Siberia

and

Mongolia south of the Stanovoi range, has a

mixed

ISTaiad fauna

whose

characters partake of the Pahearctic and Oriental provinces. Vnlo pictorum, aspecies

common

to all

Europe and

Siberia, is found at Khabarovka, in the

Amoor

Talley, as well as Anodontamagnijica, Lea, A.cellensis,Scliroeter,

and

A.pJicata, Solander, whichis

synonymous

with Crisfaria discoideus of Lea, thelatter three being

common

to Cbina.^

Accordingto Middendorf,' Anodontahercule((, Middendorf, a Japan- ese species, which is a Cristaria; Unio monf/olicus,Middendorf(=t':«/o marfiaritiferus, Linnfeus'O^ sbudAnodonta cellensis, Schroeter, are found

inthe

Amoor

region. His Unio complaitatu.s,Solander, a

common New

•Englandspecies,which hecredits to Siberia,is,accordingto hisfigures,

without lateralteeth,

and

appears tobe a stunted form of Unio mar-

garitiferus.

Schrenck^' gives the following ^list of species of the

Amoor

Valley:*

*TJniomongolicus,Middendorf.

*U.{Marg.) dahttricus, Middendorf.

*Anodontaanatina, Linnn'us.

*A.cellensis, Schroeter.

Uniograjifntus. Lea.

*U. piciorum, Linnieus.

*Unio(Marg.) margaritifcrus, Liinnxixs.

AnodontapJicata,Solander.

A. magnifica,Lea.

His U7iiograyamisis certainlynot thatspecies,buta shorter,heavier shell, belonging, however, to an Oriental group;

and

the Ufiio mongo- licmismostlikelyaformof Uniomargaritiferus with imperfectlaterals.

The

southern limit of the Pahearctic Region in North

America

can notbe accurately given, but it probably extends to near theIsthmus ofTehuantepec,as oneofthe

common

Oaliforniau

Anodontas

has been found in Oaxaca.

In allthis vast area there areperhaps not uiorethan50valid species ofNaiades, which belong to the genera Unio, Leguminaia, Anodonta.,

and

the species of Cristaria alluded to, though the

new

school of conchologistshaveconsiderably multiplied thegenera

and

have runthe specific

names up

^{into the thousands.}

The

species are, forthe most part, smallto

medium

insize,withoutconspicuous sculptureor angles, or, as arule,

any

bright patterns of coloring.

The

group of

Anodontas

typified

by

the well-known A. cygnea, Liu- nfeus,is distributed over this entire region, all the forms found in the Pacific drainage of North

America

either belonging to it or being, I think, closely related.

One

species, A. yukonensis, Lea, from the

Yukon

River, Alaska, is absolutely identical with specimens of Ano- 'Onthe authority of Moussou(Journ.deConch., XXVII,p.26).

^Malacoz. Rossica, Sib.Keise, 1847-1851,p.273.

sReiseu undForsch. imAmur-Lande, 1854-1856, p. 694.

•Thosebelongine,properlytothe palearctic regionIhavecharacterizedby an^*;the othersareOrientalspecies.

329

doiita hcr'nuliana, MicUlendort", collected

by

J)r.Dall atPetropaulovski, Kaiiichatka. This I have verified

by

comparing- the types with Dr.

Dairs^shells.

Inio ravistissus, Kobelt, of Afg'hauistan, appears to be a

member

of

one

of the great

European

groups.

Two

Uuioshave beencredited to Oregon, U.famelicus of Gould,

and

U. oregonensis of Lea.

The

typesof both^of these species are inthejSTational

Mnseum

collection,

and

I can say without hesitation that the former is a

young

shell of U. multi- striatKfi, Lea, of Brazil, while the latter is only an old, rather large

and

solid T\ roirelli, Lea, of Central America. Unio marfjaritiferiis, Linnauis,is the only species of the genus

known

at present in the Pacificdrainage of i^TorthAmerica.

Although there is a slight mingling of the forms of this

and

the Oriental regionsin the

Amoor

Valley

and

northernJai)au,I only

know

ofone group,represented

by

asingle species, belonging to the Pahe- arctic i)rovince which is extralimital, this being Unio margaritiferus, Linnaeus, which isfoundin the

Upper

Missouri of theMississippi area, and in eastern

Canada and New England

of the Atlantic drainage.

Of

^its distribution,

more

will hereafter be said.

On

the other hand,

Ido not

know

ofasingleNaiadbelongingto

any

otherprovince,which

isfound within thisgreatregion.

The Ethiopian Region.

—

All the continentofAfrica lying south of the Desert of Sahara, including the Nile to its mouth, is peopled

by

a

common

assemblageofNaiadlife.

The

onlygeneraoftheTTnionid<e rep- resented in this region are Unio, which is distributed over the whole

territory, and Biirtonia (if it bea valid genus), with afew speciescon- lined, so far asis

known,

to the region of the Great Lakes. All the Uuios aresmall to

medium

in size,

and

arenotparticularly striking in

any

way.

A

large proportion of

them

are

more

or less covered with slight zigzagor reticulated delicate sculpture,

and

in this particular,as wellasin form

and

texture,they recall the LTnios of India. This is especially true of the forms

known

from the

Cape

region.

A

few species which 1 havenot seen,

have

been reported on ratherdoubtful evidencefrom Madagascar.

Within this area are found five genera of the Mutelidie: Mutela,

^patha,

and

Pleiodon, havingawidedistribution,

and

Brazzmi

and

Cke- lidonopsis, which areprobably

more

restricted. Little is

known

asyet of theNaiadesofthis greatterritory, but longago^it

was

remarked

by

Morelet that the fauna, including the land

and

fresh-watermollusks of thisentire region,

was

remarkably homogeneous. Several of thegroups ofUnio and of the Mutelidaj appearto bedistributed over the greater part of the province. So far as I know, no species or group of the Naiades belonging withinitis foundoutside of the region, noris there an immigrantfrom

any

otherareawithin itsborders. Tlie ocean

and

the Desertof Sahara appearto be absolute barriersto the ingress or egressof Naiadlife.

The Oriental Region.

—

All that part ofAsia lyingsouth of the great

330 THE PEARLY FRESH-WATER MUSSELS—

SIMPSON. ^vol.xviii.

Thibetan plateau, including, probably, the Indus on the west

and

the

Hoang-Ho

on the northeast, ^is inhabited

by

a peculiar Unioue fauna.

With

thisregion

must

beincludedJapan,Ivorea,Manchooria, Formosa, thePhilippines,

and

probablyallthe islands ofthe

Malay

Archipelago, whicharepeopled with Naiadlife,to

and

including theSolomon group.

The

genus Unio is everywhere

abundant

throughout this area,

and

Pseudodonis

common

to nearly allof it.

A

magnificent set of

Ano-

dontasis developed in northern China,

and

in thisregion Cristaria, Lepidodesma

and

Arconaia are found. Solenaia inhabits the greater partofthe area.

Dr.

Lea was

led to believe that

two

or three of the Unios of the southern part of thisregion were found in Australia, but laterhe

was

convinced that this

was

an error,

and

thatno speciesof thetwo fam-

ilies is

common

to the Oriental

and

Australia]) regions.

The

Xaiad faunaof this region ismagnificent and diversified,

and

almost rivals thatofthe Mississippi Valleyin vigor,size, solidityandvarietyofforms.

Both

Dr. C. A.

White and

von Ihering believe that the Unios

and Anodontas

of this area are closelyrelated tothose of the centralpart of North America.

Not

onlydoes there seemtobea general relatiou- shij)

among

a large

number

of the Naiadsof this provincewiththose ofthe Mississippi basin, but several Oriental groupsare apparently so close to those ofour

own

regionthatitiswell-nighimpossibleto sei)a- rate them. Thus, the Asiatic

Anodontas

^typified

by

A.icoodiana. Lea,

il found in the UnitedStates, would be placed

by most

studentswith A. plana; theChinese Uniosofthe groupof U. housei,Lea,

and

myers- ianus, Lea, are evidently quite near the Alatus assemblagej Unio SKperhus, Lea, is very

much

likeour f7. capax, Green,

and

a

number

of the tuberculateformsofChina could almostbeplacedin the

American

groupsof U. lachrymosus

and

U. pustulosus.

Certainpeculiaritiesofshellgrowthareremarkable

among

theNaiades of thisentire region.

One

of theseis thelossor partial degeneration ofthehinge teeth,

and

anotheris theremarkable developmentof ver- ticaltooth striation,to both ofwhichattention has alreadybeen called in thispaper.

The

third isthe singular contortion of

many

ofthe spe- cies,ofwhichthere are threevarieties.

The

first

and

simplestisa

mere

bendingof the posterior part of the shell, either to the left or right, somethinglikethat of a Tellma^ whichis seen in

two

orthreegroupsof elongatedChinesespecies.

Some

oftheseforms arebentinto a strong curve.

The

second is a twisting of theshell on its axis, whichoccurs intheArconaias

and some

oftheUnios.' These

two

formsof distortion

may

occurin the

same

species.

The

third

and

most strange form of irregulargrowth is seen in a

number

of very solid, oval

and somewhat

^Arconaia provancheriana, Pilsbrr, which ^is twisted on its axis like a Parallelo- pipedon,isno doubt a distorted formof Vnio complanatus, Solander,from Canada, anddoesnot come from China,ashasbeensurmised. (See Naturaliste Canadien, XIX, p. 171, 1889.)