294 



ROHERT TRACY JACKSON ON THE 



lost nearly all the chai-ncterislies in the shell of the Gastero[)od form. INLirketl nepionic 

 and nealogic stages of growth are a striking featui-e of many Gasteropods as seen in 

 Yetus, Cyprasa, Strombns and Pteroceras. These and other stages 

 are also strongly marked in many Cei)halopods, where they are de- 

 scribed by Professoi" Hyatt (34 and other ])aj)ers). 



Kphchollc period. That period best characterized by saying that in 

 it tile aduh characters find fnllest expression; ,it is often separable by 

 marked stages from the earlier nealogic ])eriod, and also fi-om the later 

 or senile ])eriod. As examples we vvonld refer to the adnlt (middle 

 age) condition of the fignres i-eferred to as illustrating the nealogic 

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

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

 ])eriod. In Ilhiniies cortesi, PI. xxvi, figs. 3—4, the ostreaform stage 

 is ephebolic and in Vermctus radicula, fig. 9, the vermifoi-m stage is 

 the ephebolic period. 



Geratologic period. The jjcriod of decline of the individnal, often 

 marked by distinct stages. The geratologic pei'iod is subdividc'd by 

 Professor Hvatt into the clinologic and nostologic ])eriods, signifying the early and 

 later periods of decline of the individnal. As examples of geratologic stages, I would 

 refer to Venus Tridacnoides from the Miocene Tertiary of this country and to Cardita 

 planicosta, Say, from the Eocene Tertiary of Claiborne, Ala. In both foi-ms 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 a|Ti)earance. In the old age of a 

 IJrachiopod, Atri/pa rayosa, may be observed a most marked change; the regular even 

 plications of the adult become irregular and s])rea(l out in a fan-shaped lashion. This 

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

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



Fio. 9. Vermetits rndi 

 cuhi. St. (After Vur 



i-ill.) 



IV. Ostkea: Development of the soft pakts. 

 Embryology. 



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

 Dr. R. Ilorst on the European oystei-, O. edulis, L., and Professor W. K. Brooks on the 

 development of our oj'^stei", O. virginiana, Lister. The later stages of development have 

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

 fessor J. A. Ryder and the author in oui" species. 



The European species, Ostrca edidis, L., is hermaphroditic according to European in- 

 vestigators and Professor Ryder, and the embryos undergo considerable development 

 in the gill chambers of the parent. Our species, O. virginiaua, Lister, on the contrai-y, 

 is not hermaphroditic, according to Brooks (9), and Ryder (01), and differs from 0. edidis 

 in that the eggs are discharged from the j)arent and undergo their whole development 

 in the sea. It jirodnces many more eggs than the Eiu'opean species, and the embryos 

 are nnic-h smallei' than those of O. edidis. 



