94 A, E. Yerrill — Molluscan Archetype. 



This is, however, an exceptional condition in veligers, whether 

 of gastropods, bivalves, or pteropods, and therefore cannot be 

 rightly regarded as archetypical. 



The allied and still more peculiar Aplacophora (fig. 3) must 

 be regarded as degenerate mollusks that have taken on worm- 

 like forms adapted to their worm-like mode of life, burrowing 

 in the mud. In consequence of this, they have lost the cephalic 

 I !| • sense organs, shell, etc., and the foot and odontophore have 



I become nearly abortive. This group, though lowly organized, 



certainly cannot be regarded as a very primitive one, but it 

 may have been derived from the primitive chitons (Placo- 

 I phora), by degeneration. 



' ! The latter, although not to be regarded as archetypical for 



, i mollusks in general, nor even for gastropods, as a whole, must 



be considered a very early offshoot from the gastropod stem, 

 and as a group which has developed in a very divergent line.* 

 Fossil chitons occur as early as the Lower Silurian period, but 

 normal gastropods of many kinds are abundant in much earlier 

 periods. A considerable number occur even in the early Cam- 

 brian. . Of these several are limpet-like forms, and therefore 

 were even then much modified from the real primitive forms, 

 for all limpet-like living forms are known to have been derived 

 from spiral shells, and still have a more or less spiral shell in 

 the veliger stage. (Fig. 8.) 



Yarious well developed bivalves and cephalopods also occur 

 in the Lower Cambrian rocks. Most of the classes and many of 

 the orders and families had already been well differentiated 

 before these earliest known fossiliferous rocks were deposited. 

 Therefore we can hardly expect to ever find actual molluscan 

 archetypes as fossils. Some of the very ancient fossil gastro- 

 pods were, however, closely allied to certain living genera, 

 such as Pleurotomaria and Fissurella. In these there are 

 two anterior paired gills, two auricles, and several other very 

 primitive characters. This group, in fact, must be regarded 

 as more primitive even than the chitons, and therefore nearer 

 to the true gastropod archetype. Unfortunately we do not 

 know the larval stages of Pleurotomaria^ but allied genera 

 have veligers of the ordinary form. 



But for archetypical forms resembling those from which the 

 several classes of mollusks may have been developed in pre- 

 Cambrian ages, we must, I think, look among the early larval 

 stages of existing Mollusca, as already suggested. From the 

 simpler forms of the veligers and subveligers of gastropods 

 and pteropods we may derive our clearest ideas of the earliest 

 forms of primitive molluscan life (figs. 4 to 15). 



* Several prominent malacologists have recently considered the group (Isopleura) 

 including these two divisions, as a class distinct from Gastropoda. I prefer, for 

 the present, to consider the group as a sub-class. 



