320 
exists . . . . which is referable in its inherited form to Aviculopec- 
ten, and finally the true Pecten features characteristic of the adult... . 
are established. The geological sequence of these several groups is in the 
order indicated by the development of Pecten. 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 commonly 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 name 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 existing 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 /usus, ete., 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 Natica. 
, In the majority of cases the initial whorl is minute, while the 
succeeding ones enlarge gradually and 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. 
“Irom 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 
remota Billings, one of the earliest coiled gastropods of the Htcheminian 
