INVERTEBRATE PALAEONTOLOGY. 11 



1. ostrea, Linn, (typical). 



Shell variously shaped, and merely with concentric imbricating 

 marks or lamina 1 of growth, or sometimes rather obscurely costate, 

 the costa? not extending to, or strongly plicating the tree margins. — 

 O. edulis, Linn. 



2. alectryonia, Fischer (— Lopha, Bolten ; and Dendrosirea, Swainson). 



Shell often elongated and strongly arcuate laterally, with hinge- 

 line more straight, extended, and lateral margins on each side of the 

 beaks more or less dilated; surface strongly plicated, the plications 

 imparting a distinct zigzag appearance to the free margins, and very 

 rarely bearing slender projecting processes ; muscular scars eccen- 

 tric. — O. cristagalli, Linn. : O. carinata, Lam. 



3. GRYPHiEOSTREA, Conrad. 



Shell thin, elongate, straight, narrow ; lower valve rather deep 



and smooth ; upper valve flat or slightly concave, and ornamented 



with distant, regular, thin, concentric laminae ; beak of lower valve 



contorted, or turned to one side; cartilage-pit narrow, oblique.* — 



Gryphcea vomer, Morton (sp.). 



The genera Gryphcea, Lamarck, and Exogyra, Say, are included by some 



authors as sections of the genus Ostrea; but, although it is sometimes very 



difficult to decide to which of these genera certain forms of these shells 



should be referred, they can generally be readily separated into the three 



groups, and this arrangement has been found very convenient. Both of the 



latter genera usually have a more regular subovate or orbicular form than 



the oysters, and more frequently became free at a very early stage of growth ; 



the umbo of the under valve being thus generally less distorted, or often 



without visible scar of attachment. In Gryphcea, the surface is generally 



smoother, and the beak of the under valve nearly always differs from that of 



Ostrea in being more produced and curved upward, or sometimes even 



spirally twisted ; while that of its upper valve is more frequently truncated. 



* Mr. Conrad did not publish a diagnosis of this type, but merely gave the name m a list of fossils. 

 At my request, however, be gave ine iu manuscript the above diagnosis, and mentioned the above type. 

 I would add, that in perfectly-preserved specimens, the typical species, presentstbe singular peculiarity of 

 throwing out long, slender, auricular appendages (one on each side) from the lower valve uear the beak. 

 These being very fragile, are nearly always broken away, as the specimens are found ; but I observed 

 several with more or less of them preserved, in the New Jersey beds ; and one I found growing in the 

 inside of a Grijphosa vesicularis with them perfectly preserved, and apparently attached to the Grypli&a 

 by their extremities. This type or section might, with almost equal propriety, be placed as a subgenus 

 of Exogyra 



