292 



ROBERT TRACY JACKSON ON THE 



The protoconch in Gastei-opocls is represented by a spirally twisted embryonic shell.^ 

 Examples: Sycotopus canaliculatus. Gill, PI. xxiii, fig. 19. Older specimens than this 

 from Qgg eases show a well-marked protoconch at the tip of the conch and I have also 

 observed a well-defined jirotoconch in 3'onng specimens of Fulgur carica, Con., Pur- 

 pura lapillus, Ijam., Vermetus radicula, St., Buccinum undatum,Ij., and Crejndulafor- 



Figs. 1-3. Ortlioreras elegans, Miinst. Apex of conch 

 and protoconcli, viewed from the side, below and In front. 

 Figs. 4-5. Apex and protoconch of another specimen. 



Figs. 6-8. Orthoceras polihijn, KVipst. Apex of conch 

 and protoconch viewed from tlie sides and l^elow. 



a, protoconch; e, apex of conch; b, shonlder of apex. 

 All from the Triasslc of St. Casslan (after Hyatt). 



nicata"- PI. xxin, figs. 21 and 23. Eolis has been figured with an embryonic shell and 

 it is important that this genus which possesses no shell in the adult has a typical, spiral 

 protoconch when young. A very interesting condition is that which I recently found in 

 the young o^ Acincea testudlncdls \i\r. alveus, Verrill, PI. xxiii, fig. 24, which I collected 

 at Vineyard Haven, Martha's Yineyard. No protoconch exists at the apex of the conch; 



' The embryonic shell or protoconch of Gasteropods Is 

 commonly spoken of by embryologists as a "Nantilold" 

 shell; but this is not strictly correct. A Nautiloid shell 

 is spirally coiled in one plane as in the Nautilus from 

 which the name is borrowed ; but the protoconch of Gas- 

 teropods, in all the numerous cases which I have seen, 

 either as specimens or fiifures, is coiled in a spiral plane 

 and should not be confounded with tlie Nautiloid form 

 from which it is totally distinct. 



- The spirally twisted protoconch of Crepidula proves 

 that the almost conical form of the adult is not due to 



a primitive, simple condition in which the spiral has not 

 .vet become developed ; but rather to a moditied condition iu 

 which tlie ancient spiral form, shadowed in the protoconch, 

 is almost entirely lost in the adult. Similar observations 

 have been made on Flssurella in an exhaustive paper by 

 M. Boutan. Tlie protoconch is very sharply defined and 

 is beautifully clear in the young of Cre.piilnla fornicata. 

 Lam., and C. plana, Say, both of which species are com- 

 mon on our shores and may be collected iu abundance In 

 July and August. A short paper on this shell is now in 

 jireparation for publication. 



