Jackson.] 546 [April 4, 



REMARKS UPON THE ADULT AND ALLIED GENERA. 



Professor Ryder (21) mentions that he has found prodissoconchs 

 like that of 0. virginiana in several species and thinks it is prob- 

 ably characteristic of the family. I have found prodissoconchs pre- 

 cisely like that of 0. virginiana in other living and many species 

 of fossil oysters, in several species of Exogyra and Gryphsea, from 

 the Jurassic and Cretaceous, and in some related genera. 



In oysters as far back as the Jurassic, species have been found 

 with the spat growth marked off into five stages similarly to Ostrea 

 virginiana, showing that the series of stages found in our species 

 already existed at that early date in some members of the family. 



It is not held that the spat stage is in all forms divisible into 

 five stages ; but only that it is a persistent characteristic found in 

 some species. In all oysters, living and fossil, the silphologic 

 (spat) period is more or less clearly marked. In species which are 

 plicated or otherwise peculiar, the adult characteristics do not often 

 appear before the close of the spat stage. 



The genus Gryphsea is described as free or attached in the young 

 stages, yet I have not been able to find a species in which it was 

 free at a very young stage. All have a flat area in the lower valve 

 and a curved in the upper, corresponding to the attached spat stage 

 of the oyster. G. arcuata of the Lias would be commonly consid- 

 ered as free, yet I have found well preserved specimens in which 

 the lower valve is flat at the apex, where it was attached when 

 3'oung, and correspondingly the upper valve has a small convex 

 area at the umbo, as noted by Sowerby (27). 



The area of attachment in many species of Gryphsea is uniform 

 in size, indicating that the shell dehisced at a very definite period 

 from the object of attachment. In fact, highly arcuate forms must 

 have separated early, as the form could not have been developed, 

 unless the shell was free. Other species were attached for a very 

 variable period, as is seen by the differing size of the flat area of 

 the lower valve and corresponding curved area of the upper. This 

 is well shown in any large collection of the European form of Gry- 

 phwa vesicularis. 



Exogyra, also, is said to have been free or attached ; and here 

 the case is somewhat more complex. Take the typical E. costata 

 of the Cretaceous, for example. Specimens are found in which a 

 flat area at the apex of lower valve distinctly shows them to have 



