204 



ROBERT TRACY JACKSON ON THE 



lost nearly all the characteristics in the shell of the Gasteropod form. Mai-ked nepionic 

 and nealogic stages of growth are a striking feature of many Gastei'opods as seen in 

 Yetus, Cyprsea, Strombns and Pterocei-as. These and other stages 

 are also sti-ongly marked in many Cephalopods, where they are de- 

 scrilicd by Professor Hyatt (34 and other papers). 



Ep]i(holic j)eriod. That period best characterized hy saying that in 

 it the adult characters find fullest expression; it is often separable by 

 mai'ked stages from the earlier nealogic period, and also from the later 

 or senile period. As examples we would refer to the adult (middle 

 age) condition of the figures referred to as illustrating the nealogic 

 period and also the next or geratologic period. In Slava fibrosa, PI. 

 xxni, fig. 12, the radially plicated area is referable to the ephebolic 

 period. In Hinnites cortesi, PI. xxvi, figs. 3-4, the ostrcaform stage 

 is ephebolic and in Vermetus radicula, fig. 9, the vermiform stage is 

 the ephebolic period. 



Geratologic pmod. The period of decline of the individual, often 

 marked by distinct stages. The geratologic period is subdivided by 

 Professor Hyatt into the clinologic and nostologic periods, signifying the early and 

 Liter periods of decline of the individual. As examples of geratologic stages, I would 

 refer to Venus Tridacvoides from the Miocene Tertiary of this country and to Cardlta 

 jjlanicosta, Say, from the Eocene Tertiai-y of Claiborne, Ala. In both Ibrms old age is 

 strongly indicated by a loss of costations and other characters found in the ephebolic 

 period, the new period assuming a strikingly different appearance. In the old age of a 

 Brachiopod, Atryj^a j-i/f/osa, may be observed a most marked change; the regular even 

 plications of the adult become ii-regular and spread out in a fan-shaped fiishion. This 

 is shown in many of M. Bari-aude's figures and is more or less completely developed in 

 some specimens of any large collection of American or European specimens. 



Fig. 9. Vermetus rndi- 

 cula. St. (Aflfi- Ver- 

 rill.) 



IV. Ostrea: Devki.opment or the soft parts. 

 Embryology. 



The early embryonic development of the oyster may be followed in the writings of 

 Dr. R. Ilorst on the European oyster, O. edidis, L., and Professor 'W. K. Brooks on the 

 development of our oyster, O. inrginiana, Lister. The later stages of development have 

 been described by Professoi" Huxley and Dr. R. Horst in the European oyster, and Pro- 

 fessor J. A. Ryder and the author in our species. 



The European species, Ostrea edulis, L., is hermaphroditic according to European in- 

 vestigators and Professor Ryder, and the embryos undergo considerable development 

 in the gill chaml)ers of the parent. Our species, O. virginiana, Lister, on the contrary, 

 is not hermaphroditic, according to Brooks (9), and Ryder (Gl), and differs from O. edidis 

 in that the eggs are discharged from the parent and undergo their whole development 

 in the sea. It produces many more eggs than the European species, and the embryos 

 arc much smaller than those of 0. edulis. 



