1888.] 547 [Jackson. 



been attached when young, but rarely are adults found with the 

 object of fixation still clinging to them, showing that as in Gry- 

 phaea, and many oysters, they separated naturally. Other speci- 

 mens are found, which, upon most careful examination, show no 

 flattened area on the lower valve. I have already noted that Os- 

 trea may in its early silphologic stages be attached, and yet not 

 conform to the object, or flatten at all, so that if it could have sep- 

 arated at the end of this period, and lived, the adult would show 

 no sign of early fixation. This is just what I think took place in 

 specimens of Exogyra which show no flat area. In such specimens 

 there is no proof that they were not fixed during the young stages ; 

 but merely that they dehisced before the shell conformed to the 

 outline of the object of support. 



In the Ostreadas there is a striking peculiarity of the adult which 

 I believe not to have been thus far noticed. The two valves are un- 

 equal, one being concave and the other flat ; but they are not only 

 unequal, they are ver,y dissimilar, as different as if they belonged 

 to distinct species in what would be considered typical forms. As 

 examples we may take Ostrea edulis of Europe, 0. compressirostris, 

 Eocene, of this country, Exogyra costata, Cretaceous. In these, 

 the lower valves are plicated, and the upper smooth, and regular- 

 growing, without plications. The examples could be multiplied ex- 

 tensively, but these suffice to give my meaning. This is a highly 

 interesting feature, as it is evidently a case of inherited or acquired 

 characteristics, finding very different expression, in the two valves 

 of a group, belonging to a class typically equivalvular. As might 

 be expected, variations from this rule are common, as in Ostrea 

 marshii, 0. larva, etc., where the valves are closely alike. Dis- 

 similarity seems, nevertheless, to be the rule. Ostrea virginiana 

 is a good example of a form in which the upper valve sometimes 

 reflects the form of the lower, but this is commonly departed from, 

 and proves the rule, by dissimilarity being its typical preponderat- 

 ing form. 



In adult Ostrea virginiana, the left valve is concave, and the 

 right valve is flat, when growing naturally, the left valve being- 

 lowermost. Professor Ryder notes that the same relative shape is 

 held by oysters which grow vertically in crowded beds. Oysters 

 frequently attach themselves to the lower side of objects, so that 

 the left valve is uppermost, and right valve lowermost, the reverse 

 of the common position, fig. 19, pi. vn, and here again the normal 



