
918 THE AMERICAN NATURALIST. [Vor. XXXVI. 
The Protoconch of Gastropods. — The apical whorl of a gas- 
tropod shell has come to be generally known as the protoconch, 
though this term had been preoccupied for the corresponding 
apex of cephalopod shells. The protoconch of gastropods is 
essentially different from that of cephalopods in that it is (at 
least in the majority of types) twisted into one or more coils 
of a spiral nature, whereas that of cephalopods, so far as 
known, is a mere swollen bulb. The earliest portion of the 
gastropod protoconch agrees essentially in form with that of 
the cephalopods (Spirula, Ammonoidea) and pteropods, but 
spiral coiling appears very early in the majority of forms. 
Thus the protoconch of the Gastropoda may on the whole be 
considered as more specialized than that of the Cephalopoda. 
It might perhaps be thought desirable to dignify this specialized 
type of protoconch by a distinct name, as has been done by 
Jackson in the case of the pelecypod protoconch (* prodisso- 
conch"). If so, the name “ protorteconch," suggested to me 
by the late Professor Hyatt, would be most applicable. The 
shell or conch of gastropods may be specifically described by 
the term ** torteconch," a name also used for the spirally coiled 
shell of cephalopods (Turrilites, Trochoceras). 
The characteristics of the protoconch of most living gas- 
tropods are very variable, as might perhaps be expected in a 
class whose living members are on the whole highly specialized. 
Even in Tertiary times this specialization 
of the protoconch is noticeable. It is not 
always possible to determine the precise 
line. of demarcation between protoconch 
Fic. 1.— Rhopalithes rugoides and conch, since in a large proportion of 
fosoid (bulbow) premens types the two grade into each other imper- 
with riblets on last portion ceptibly. In certain types (Buccinum, etc.) 
the conch. Eocene. Paris à line and a slight change in the growth 
of the shell indicate the place where we 
may most reasonably make the separation between protoconch 
and conch. In a number of cases (Fusus, Hemifusus, certain 
Murexes, etc.) the end of the protoconch is strongly marked by 
the existence of a pronounced varix and an abrupt change of 
ornamentation (Fig. 5). In the majority of cases, however, no 

