10 



GENESIS OF THE AEIETIDvE. 



chamber perhaps longer in proportion. The entire absence of a caecum, and of 

 all signs of a siphon, may be inferred with probable certainty in this first stage ; 

 and we proposed, in the paper referred to above, to name it the Asiphonula. This 

 form may indicate the previous existence of a common univalve ancestor for the 

 Cephalopoda which resembled the Pteropoda. Certainly the aspect of the calcare- 

 ous protoconch of Ammonoids and Belemnoids favors this idea, first suggested by 

 Von Jhering ; and the asiphonula adds another argument, since it has no siphon or 



true septum. The young of the 

 Pteropoda, especially the ancient 

 forms, had calcareous protoconchs 

 in most forms; but doubtless there 

 are more primitive shells in which 

 the protoconchs had the more 

 primitive, embryonic, conchioli- 

 nous stage of development. The 

 evidence, therefore, is not conclu- 

 sive, but it justifies the supposition 

 that Cephalopods and Pteropods 

 had originally some common an- 

 cestor, a true shell without septa 

 or siphon, and possessing a proto- 

 conch, which might have been 

 conchiolinous. 



There is, however, another 

 group, the Scaphopoda, which 

 may claim to be considered in 

 this connection. According to 

 W. K. Brooks, the veliger is rep- 

 resented by the adult of Dentalium 

 in several of its leading charac- 

 teristics, and this must be regarded 

 therefore as the most generalized 

 type of the true Mollusca. It is 

 quite possible that the asiphonula 

 may have retained some of the 

 characters of the veliger, and may 

 have resembled Dentalium or 

 some common ancestor, and may have descended from this form without having 

 passed through any pteropod-like ancestral modification. The peculiar resem- 

 blances of the young of some of the Goniatitinse and the adults of Tentaculites 

 among Pteropoda may be entirely due to homoplasy, and not to homogeny. 1 



1 These terras were first used by Lankester (Jour. Micr. Sci., XVII., 1877, p. 436). They express 

 phenomena with which naturalists have long been familiar, "homoplastic" meaning representation and 

 independent origin of similar characters, and " homogenous " meaning genetic connection. See also previ- 

 ous use of terms^ Heterology and Homology for the same phenomena by Cope, in his masterly essay, " Origin 

 of Genera," Proc. Acad. Sci. Phila., 1868, and " Origin of the Fittest," p. 95. 



Fig. 1-3. Apex and protoconch of Orth. eJe/jans, MUnst., from 

 the side, below, and in front. In Fig. 2 the fine striae really 

 cross the shoulder of the apex (b), and reach to the proto- 

 conch (a). Named by Klipstein, Loc. St. Cassian, Coll. Brit- 

 ish Museum. 



Fig. 4, 5. Apex and protoconch of another specimen mounted 

 witli the first on the same card. Kamed by same, Loc. same, 

 Coll. same. 



Fig. 6-8. Views from the side, front, and below of the same 

 parts in Ortli. politum, Klipst. The shading on the protoconch 

 of Fig. 8 does not indicate structure; this protuberance is 

 smooth, a, protoconcli ; b, shoulder of the area of the cica- 

 trix. Named by same, Loc. same, Coll. same. 



