Living gastropods are classified based on their morphology, mainly the anatomy of their soft parts. The exception to my original statement that the classification of Gastropoda is primarily the work of neontologists is the work of.
PROPOSED CLASSIFICATION
It will also be noted that two of the three primitive superfamilies of the prosobranch Archaeogastropoda, the Mac-. These give us some clues about the morphology of the soft parts and about the physiology and embryology of the primitive proso branches.
EXPLANATORY NOTES
13 PRIMITIVE FOSSIL GASTROPODS — KNIGHT 7 Chronogenesis and extent in time.—To give an umbrella. It will be noted that the most important dichotomy in time (as well as in morphology) between the Isopleura and Anisopleura is in the early Cambrian, at the beginning of the fossil record.
SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. II
Illustrations.—In addition to certain diagrammatic drawings to illustrate various points under discussion, I have included drawings of generalized restorations of a number of characteristic Paleozoic genera mentioned in the text. In the text, references to the list are cited in brackets according to author and date.
ACKNOWLEDGMENTS
ARGUMENT
NEONTOLOGICAL CONSIDERATIONS MORPHOLOGY OF LIVING POLYPLACOPHORA
The numerous paired ctenidia appear to be metameric repetitions of a primary pair located on either side of the anus and. The nervous system is not convoluted and shares the bilateral symmetry with the rest of the body.
MORPHOLOGY OF LIVING PLEUROTOMARIANS
In the Bellerophontacea, which are supposed to be the immediate forerunners of the pleurotomarians, there is likewise a single pair;. Close to the inner end of the slit (or row of tremata) is the anus at the distal end of the anal tube.
ANISOPLEURAN ONTOGENY
Although the organs of the pallial cavity have not yet appeared when torsion begins or are extremely rudimentary, they eventually mature into an anterior position after torsion, even though their primitive position should have been posterior. Torsion naturally affects the relationship of the shell to the head and foot so that its apex points towards the back of the head instead of forward.
PRELIMINARY INFERENCES FROM NEONTOLOGICAL CONSIDERATIONS
To the left of the figure is the ciliated velum, the cup-shaped shell is above and the rudimentary foot below. The digestive tract is dotted with the mouth below and to the left and the anus high and to the right of the figure. The mantle cavity has appeared and with the anus turned slightly to the right of the animal.
They would have retained other peculiarities of the parent stock, such as the basic ornamentation plan. That is, it would have no anal emargination in the lip of the shell.
RECAPITULATION
Consequently, we should not be surprised to find that lateral asymmetry appeared in the paleontological record shortly after the establishment of a lineage of isostrophic gastropods (bellerophonts) with only torsional asymmetry. Since this most distant stage was, according to the hypothesis, an ancestor of both the Polyplacophora and the Anisopleura, it could possibly exhibit characters fundamental to the transverse segmentation of the shell into separate plates, characters such as multiple transversely paired ones. The anal tube emerging from the stomach passes through the pericardium and ends near the outlet and between a pair of ctenidia.
It is thought to have closely resembled primitive pleurotomarians, but with complete lateral symmetry. It may have possessed in a rudimentary form traces of the tubules that carry the aesthetes in the tegument.
PALEONTOLOGICAL CONSIDERATIONS
No Platyceratidae are known from rocks older than the Middle Ordovician, and no Capulidae in the Paleozoic. The anal spur is a deep, rounded sinus in Sinuopea and a deep angular sinus in Schisopea and Dirhachopea, perhaps culminating in a short notch-like fissure in the latter. In addition to the bellerophont genera discussed, three genera of macluritoid gastropods occur in the final phase of the Upper Cam-.
Of these, Helcionella (Fig. i, Fig. 2) and Scenella Billings, 1872 (Fig. i, Fig. i) both appeared together with Oelandia (Fig. i, Fig. 5) in the Lower Cambrian and are thus among the earliest known snails. Although not yet described or predicted in the literature, chitons (Polyplacophora) are known from Upper Cambrian strata of the Trempealeauan stage.
CLIMBING DOWN THE FAMILY TREE
One is Cycloholcus from the base of the Upper Cambrian-Dresbachian stage and the other is Coreospira (pi. i, figure 7) (both mentioned earlier) from near the Middle and Lower Cambrian boundary, probably at the top of the border. The shell of the other genus, Helcionella (pi. i, fig. 2), is also cup-shaped and includes species that are low and wide, as well as species that are high and narrow. Specimens of an Ordovician genus of this family, Archaeophiala^^ Perner (pi. i, fig. 3), beautifully preserve the muscle scars.
Typical of these early Cambrian genera are Scenella (pi. i, fig. i) and those species of Helcionella (pi. i, fig. 2) which have the low goblet form of the genotype. If we accept it as homologous, we must then accept that Oelandia (pi. i, fig. 5) has undergone torsion, but in most other respects has retained at least some of the external features of Helcionella.
EXPLORATION OF OTHER EARLY BRANCHES
Similarly, the anterior opening of the patellar scars appears to reflect the torsion of the primitive posterior pallial complex and cavity to an anterior position above the head. Although there is a very thin scar-like line in the patellas connecting the open ends of the horseshoe, it is. However, the shape of the shell is so similar to that of Lepetopsis that I will tentatively associate the two.
This sharp arc of the supposed basal part of the cliff looks like a notch keel with an internal channel. In the truly sinistral gastropod, all organs of the body are reversed in position from that of the gastropod which begins ontogenetically with the early cleavages of the egg.
38 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. II the genera dealt with above Antitrochus and Camhodgia are probably
Now, in one of the generations of Paleozoic gastropods of the group we are considering, the operculum was thick and calcified. We know of no operculum in any other genera included in the group under consideration. Assuming that we have solved the wrapping problem in the set under consideration, namely that it is dex- hyperstrophic.
But since shells are hyperstrophic and appear in the fossil record shortly before the first known pleurotomers, the asymmetry may be very different from that of the main gastropod lineage. The whorls are ovoid in section, the narrow end of the ovule being at the periphery.
TAXONOMIC CONCLUSIONS
There are also other conclusions as to the subdivisions of the next lower grade, but consideration of these is postponed until the class and the two proposed subclasses fall under Mathematics. The range of peculiar morphological modifications in this class is so great that it is extremely difficult, if not impossible, to establish a brief diagnosis covering all gastropods, without excluding some forms that should clearly be included. The return of the isopleurans to the class, however necessary it may seem, increases the difficulties, for by it we reduce the convenient criteria of torsion and of a single scale to a status diagnostic of subdivisions of lower rank.
In a few gastropods specialized in free swimming, the foot can be modified into fin-like organs, in the polyplacophores the primitive single shell has been divided transversely into eight segments, and in some highly specialized forms the shell has disappeared in the adult. They first appear in the fossil record in the Lower Cambrian rocks and continue into the present.
MAJOR DIVISIONS OF THE SUBCLASS ISOPLEURA Order Polyplacophora. — Isopleuran gastropods with the shell
In the Silurian, what appears to be a new genus hitherto unrecognized is represented by Palaeacmaea. Muscle scars (observed in Tryblidium, Pilina, Drahomira, Prop-Una and partly in Cyrtonella) are essentially similar to those of the preceding family; concentric decoration or radiating decoration. 1^The muscle scars of Prolina cornutaformis (Walcott), its genotype and the only species referred to the genus in the published literature, are unknown.
Ulrich and Josiah Bridge are available to me and several species referred to the genus show them clearly. Besides reviving Lankester's subclass Anisopleura, which without the Monoplacophora (Tryblidacea of Wenz) is equivalent to the class Gastropoda of Wenz, 1938, I now propose some changes.
APPENDIX
INTERPRETATION OF THE BELLEROPHONTS AMPHIGASTROPODA VS. PROSOBRANCHIA
Feeling uncertain about his interpretation of the bellerophonts, he quite rightly looked for corroborating evidence, and he felt that the discovery in the bellerophonts of multiple, paired dorsal muscle scars like those of the triblidians would be strong supporting evidence, as indeed it was would be Feeling that his views on the close relationship of the bellerophonts and tryblidians were fully justified, Wenz published his paper which gave systematic elaboration to those views by assigning them a subclass, the Amphigastropoda (Wenz, 1940) .^°. Both of these facts seem to be fatal to Wenz's arguments about the closeness of the relationship between the tryblidians and the true bellerophonts.
Perhaps a diagrammatic restoration of some of the more significant soft parts with an interpretation of the flow of water in the mantle cavity will save pages of words. I have abandoned the ideas expressed in Figure 7a-c as untenable in the light of more accurate knowledge of the aeration currents in Haliotis than I then had.
I noticed that they gave a clue to the area on the edge of the mantle through which flowed the water currents that aerated and washed away the mantle cavity. The works of Yonge and Crofts on the aeration flows in several gastropods, including the pleurotomaria Haliotis, seem to strengthen the suggestion of K's channels. This derived circulation is in all respects that of a prosobranch and seems a reasonable approximation of its likely condition during life.
Just as this work reached the stage of page-proofing, a copy of the "Traite de Paleontologie", was published under the direction of Prof.
PLATES
Right side, b, view looking directly into the socket of the shell, showing the muscle scars (dark pigmented) and the 'shadow scars'. This drawing was not made based on the speci-.
34; 6 Maciuritacea
