454 CRETACEOUS PELECYPODA 



fine transverse waved lines or striae ; muscular scar sub-central, varied in shape, 

 depressed, or very little elevated ; surface concentrically lamellar, or with radiating 

 sub-obsolete ribs ; type, 0. edulis, Linn. 



la. The name Ostrea has been reserved for the species with foliated or con- 

 centrically lamellar shell, only occasionally there are some superficial ribs present, 

 but they do not extend to the margin of the shell, this being entire, though some- 

 times irregular. 



16. Alectryonia, Eisch. de "Waldh., 1807, (vide Bull. Soc, Moscou, viii, 

 p. 108; LopJia, Bolten, apud H. and A. Adams ; De^idrostrea^ ^w^itlb.^ 1840). 

 Shell generally attached near the beaks, irregular, usually of a broadly ovate, 

 or rounded, more rarely of an elongated sigmoid shape ; the posterior part of the 

 hinge-line is often somewhat expanded, the muscular scar excentric and raised, the 

 beaks slightly curved, with the ligamental groove as in Ostrea; surface with 

 radiating ribs, which form strong dentations at the margin ; type, Ostrea crista- 

 gain, Jjinne. A peculiar section oi Alectryonia are the narrow, mostly sigmoid 

 shaped, strongly costated Ostrece, represented by O. carinata, Lam. 



Species of Ostrea occur in great number in the mesozoics. Those belonging 

 to Alectryonia are there most numerous and decrease in the tertiaries, while the 

 number of true Ostrece increases from that time up to the present. 



2. GryphcBa, Lam., 1801, fFycnodonte, Fisch. de Waldh., 1835, Bull. Soc, 

 Moscou, viii, p. 118; Gryphostrea, Conrad, 1865, Am. Journ. Conch., I, p. 15). 

 Shell generally broadly ovate, inequivalve, usually attached when young, mostly 

 free when adult, attached valve larger, convex, with a strongly incurved and often 

 spirally twisted beak, smaller valve mostly flattened with a simple obtuse beak ; 

 ligamental groove simple, arched, distinct in the convex valve, fiat and often repre- 

 sented by a uniformly striated margin only in the smaller valve ; muscular scar 

 excentric, generally somewhat impressed; type, G. angulata, Lam., recent. 



The GryphcecB are first known in the Trias, and it is not improbable that 

 they are already found in the palaeozoics. In the Lias and Jura they are most 

 numerous, in the cretaceous period their number decreases ; from the tertiaries 

 there are only few known, and in the recent state only one. 



3. JExogyra, Say, 1819, (Am. Journ. Sc. and Arts, i and ii, p. 43). Shell 

 usually elongately or roundly ovate, somewhat irregular or of sigmoid shape, inequi- 

 valve, right valve usually more convex, attached at the umbo, left valve depressed, 

 or slightly elevated, beaks of both valves strongly spirally twisted; ligamental 

 groove narrow, curved, marginal, flattened out inferiorly (or on the concave side), 

 bounded above (or on the convex side) by a narrow deep groove, which always 

 remains well marked, while the upper part of the ligamental groove is in old shells 

 covered up by a shell-lamina ; both valves generally have a raised fold, or a blunt 

 tooth, at the posterior end of the ligamental groove; muscular scar excentric, 

 impressed; type, U. costata, Say, from tertiary deposits of New Jersey. 



The species of Exogyra are probably most numerous during the cretaceous 

 period; none is known" recent. There are two slightly different sections of the 



