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ROBERT TRACY JACKSON OX THE 



The protoconch in Gasteropods is represented by a spirally twisted embiyonic shell.^ 

 Examples: Sycotopns canaliculatas, Gill, PI. xxrir, fig. 19. Older specimens than this 

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

 observed a well-defined protoconch in yonng specimens of Fulgur carica, Con., Pur- 

 pura lapillus, Lam., Vermetus i-adicula, St., Baccinum undatum, L., and Grepidula for- 



FiGS. 1-3. Orthoceras elegans, Miinst. Apex of conch 

 and protoconch, viewed from tlie side, below and in front. 

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



Figs. 6-8. Orthoceras poUtiim, KMpst. Apex of conch 

 and protoconch viewed from the sides and below. 



a, protoconch; e, apex of conch; 6, shoukler of apex. 

 All from the Triassic of St. Cassian (after Hyatt). 



nicata,^ PI. xxin, figs. 21 and 23. Eolis has been fignred with an embryonic shell and 

 it is important that this genns 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 of ^cmcca testudincdis yar. alveus, Verrill, PI. xxrii, fig. 24, which I collected 

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



' The embryonic shell or ])rotoconcli of Gasteropods is 

 commonly spoken of by embryologists as a " Nantiloid" 

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

 is spirally coiled in one plane as in the Nautilus from 

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

 teropods, In all tlie luimerous cases which I have seen, 

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

 and should not be confounded witli the Nantiloid form 

 from which it is totally distinct. 



^The spirally twisted protoconch of Crcpidula proves 

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



a primitive, simple condition in which the spiral has not 

 yet bec(nne developed ; but rather to a moditied condition in 

 which the ancient spiral form, shadowed in the protoconch, 

 is almost entirely lost in the adult. Similar observations 

 have been made on Fissurella in an exltaustive paper by 

 M. Boutan. The protoconch is very sharply defined and 

 is beantifidly clear in the young of Crcpidula furnicata. 

 Lam., and (J. plana, Say, botli of which species are com- 

 mon on our shores and may be collected in abundance in 

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

 jireparation for publication. 



