320 



exists .... which is referable in its inherited form to Aviculopec- 

 ten, and finally the true Pectcn features characteristic of the adult .... 

 are established. The geological sequence of these several groups is in the 

 order indicated by the development of Pectcn. We have, therefore, a 

 clear case of the ontogeny of an individual illustrating the phylogeny of 

 the group." 



Gastropoda. — For studies of the Gastropoda in which growth stages 

 have especially been taken into consideration we are indebted chiefly to 

 Grabau (22. 23, 24. 25) and Burnett Smith (53, 54. 55, 56). My illustra- 

 tions of recapitulation among the members of this class will be drawn, 

 therefore, from the writings of these two authors. 



It is eoininonly known that the apical whorl of the gastropod shell 

 may differ materially from the succeeding portions of the shell (conch), 

 being smooth and without ornament in cases where the conch is highly 

 sculptured. or in some forms, as Acmaea and Crepidula, being coiled, al- 

 though the adult shell is patelliform and non-coiled. To this apical whorl 

 the name "protoconch" has come to be applied, a nauie which, as we have 

 already seen, is also applied to the embryonic shell of the Cephalopoda. 

 Grabau (22) has suggested the use of the name "protorteconch"' in place 

 of protoconch for the initial shell of the gastropods. 



The protoconch of the existiug Gastropoda is more variable than 

 that of the Cephalopoda, as would be expected from the highly specialized 

 nature of most of the extant representatives of the class. In most cases 

 there is no definite line of demarkation between the protoconch and the 

 conch, but in a few cases, as in Fusils, etc.. the "end of the protoconch is 

 strongly marked by the existence of a pronounced varix and an abrupt 

 change of ornamentation." (22) "The early whorls of the protoconch 

 . . . . are smooth rounded coils of the type found in adult Xaticu. 

 . . . . In the majority of cases the initial whorl is minute, while the 

 succeeding ones enlarge gradually aud regularly. In some types the in- 

 itial whorl is large and swollen This type of protoconch has 



been termed 'bulbous' by Dall (19). The naticoid form of protoconch 

 is in general umbilicated, and it is probable that at least the earlier por- 

 tion of the protoconch is umbilicated in the majority of gastropods. 



"From the characters of the initial whorls of the protoconch we may 

 argue that the radicle of the coiled gatropods must have been a naticoid 

 type with a well-marked umbilicus. Such a type is found in Straparollina 

 reraota Billings, one of the earliest coiled gastropods of the Etcheminian 



