MWTIIOD OF INVESTIGATION

The principles 1 .nvolved in a solution ot this problem have been applied in several oimile.r lrrvestigo.tlons.

They a!"t'l found for example, in the comprehensive stuay of the genus• spirj.ter, by tentorf:/, and likewise 1n

!/ ^Fenton, c. L., l?Ubl. Wagner Free Inst. soi.

₁

vol. 2, 1~31.

the · w ork by Rowe!/ on the eeh!noid genus, Micrsster. In

!/ ^{Rowe; A . •} w., ^{Qua.rt. Jour,} Geol. soc. London., vol. 55, P• 494, 1899.

this conneetion. reference should be made also to the cle$aJ.l•

ed studies ot :Mesozoi o oysters by T~ems.rJ/ ^{• and t hOs e on}

cor•l• by oax-ruther_.!/. In each or these investigations 4/ carruthe:r$, 11• G•, Quart. Jour. Geol. soc. London,

- vol. 66, PP• 523•538

₁

1910.

two tundamental features were stressed. (l) Evolutionary changes Which particular fossil forml!S undergo were used to

este.blieb biostrtltigraphio zones, t6cil1tating the c.orre•

lation ct various lithologie units. (2) The systematic elassitieation was simplified. by an elucidation of evolu.•

t!onary l119e-lh Thus torm$ w«re determined to be clo$ely

related that had previouely been assigned to new speeies

p. 7

without good reason or because 1d.ent!f1oation was 'based on single or d a maged specimens.

In the invesliigations cited above as well as in tbe present study certain qualifications must be met before the results obtained and the o o nelusions drawn c a n be regarded as jus t ifiable.

(l) The species Qt tbe genera ohosen for investiga- t:i.on. mu st be represented by an abundance of individuals at all stages throughout its geologio range. Most ot ^the

structural differences obsertred on two given specimens a.re small and - i ndi'1idu&l variations are often niialaading.

It is the average vu. riation

~OW!t

by cumulative changes

occurJ:"ing in a l arge number of individuals which 1$ found most

useful.~'''

(2) The geologic positien

~

stratigra phic ooourrenoe or individual samples must be established as a<:ourately as possible. Obviou.•lY the course in evolution of a ohar•

acter oan be d etermined only when the relative stratigi-aphic age or position or individual stage& in the modification of this teatu:re 1$ known. once the tx-end and rate or

evolution are determined• strati gre.phio OQrrelations can be established with even greater certainty.

{5) Only well ... preserved or carefully prepared speet- :mens should be used. aonclusions based on poorly preserved.

or fragmental rnaterial are often not valid since tbe evo•

lutionary changes may be .masked or oompletely destroyed.

p, 8

(4) The methods employed in making .measurements

or !n recording descriptions should be applied as

uniform~

ly as possible. constant vigilance must be exerei'sed t _· o red.uee or

~vo

1 h d um.an error. Tu.us, ^~

^RoY1~

5/ took

photom1crog;raphs ot ai)eoimens he was concerned with and me.de his measurements from the photographs to ob"liate :ml stakes.

(5) A ste.ndard basis tor com parison of different forms must be d evised, e1 th er

¹

,y direct niernaurement or through calculaticn.. !n making comparisons it is imvor- tant to utilize eharaoters independent of those w:tioh

me~,

vary among indivi. duals; as for example, tho so ot

s i

^2;e

and shape.

In the present study more them 2000 speci.m.ens were exai, J_ned, of w hich over 750 were me a sured. These peleor•

pods wer0 identitied as qlycymcris veatchU. , Q• veatoh11;

va:r. tnas1.0..!

st~nt;on,

Q• major Stanton, and .Q• :major, subsp.

me~anosensis

Clerk. Measurements were mad• or ^{the thlek•}

ness, length, and height

Ci>~

the shell, as well as ot the vddths of the ribs and of' the rib · , , . interspeoes. In eddi t1on, the number of ribs was counted, chf 1 t• net0r and amount or

concentric sculpturing on each valve were observed, d$ ....

gree or de'V'elopment of :postetior groove :noted, and the

:position of the beaks established where this was possible.

Ratios of length to t hickness, height to thic.knes:s, and

h~ight

to length were calculated in order to reduce th@se data to e:erta i n common denominators by which the oompari•

sons mi ght be mere readily comprehended. In t h1$ way,

variations which are magnified by the size of the animal come to be seen in proper perspective, while evolutionary trends are more clearly indicated. . Also, local variations due to possible environmental influenees rather than

evolution are thus more readily recognized. All specimens less than 6 mm. in thickness were considered imma ture

and c a lculated separately. This sep a ration was made (1) beo a use it beoame evident early in the investigation that a specimen with a thickness or convexity of shell ef about 6 mm. marked a rather sharp break within the growth series;

all specimens r,t le.s· s thiokn$ss tended to have much higher ratios of length to thickness and height to

thickne.s~

than did la:rger and more eonvex specimens; and (2) measure•

ments of' valves which were less than 6 mm . in thickness were subject to e onsiderable error. in some cases sutfi•

cient to invalidate the results obtained from such measure- ments.

Th~

stratigraphic succession Of the localiti · es from

which individual speoim. ens were measured w ae determined

by reducing individual shell ratios to average values for

a sing le locality. Looalities representing the same time

stages or narrow time ... strati graphic zones were com bined

p. 10

by weighing the average ratios wi tb. the number of

specimens measured from each locality. ¥ blare satisfactory stratigraphi9 1nfQrmation is available this procedure is

·simpltl, but in instances where the stratigraphy is little

kn<>Wn or unknown altogether, the relative ege ot the locality 1.s based on the apparent evolutionary stage of the individual spee1mene. Thi$, obviously• does not fur•

nish the most accurate results, . but appears to be the

only met hod available to the investiga tor under the

circumstances,

p .. l l