INVERTEBRATE PALEONTOLOGY. 27 



be found desirable, however, to modify the diagnosis somewhat, so as to 

 include some of the larger, smooth, compressed Pectens, so called, of the 

 Cretaceous and Jurassic rocks. At any rate, such shells cannot be properly 

 included in the genus Pecten, as restricted to such forms as P. maximum. It 

 is more nearly related to Amussium, but differs in its closed margins, the 

 strong concentric markings of its right valve, and the absence of internal 

 radiating costa?, as well as in its narrower form, and the shape of its ears. 

 The same characters, or at least a part of them, also distinguish it from 

 Pseud-am ussiu in. 



I am not quite sure that this genus occurs in any but Cretaceous rocks ; 

 but, as already suggested, it may possibly include some Jurassic species. 



Eburneopecten, Conrad, 1865, founded on an Eocene species (P. scintil- 

 lutits, Con.), seems to be a very closely-allied type, that may be congeneric. 

 It only differs somewhat in the form of the ears, and in wanting strong con- 

 centric markings of the right valve — both of its valves being quite smooth. 



S > 1 1 1 ■ j . ■ I o ■ 1 1 ■ 1 1 1 : i rigida, H. & M. 



Plate 10, figs. 5, a, b. 



Pecten rigida, Hall and Meek (1854), Mem. Am. Acad. Sci. aud Arts, Boston, V, 381 (n. s.), pi. 2, figs. 4, a, b, a 



(not Pecten rigida, Sowerby, 1818). 

 Pecten Maliii, Gabb (1861), Catalogue Cret. Foss., 158. 

 Syncijclonema rigida, Meek (1864), Smithsonian Check- List Invert. Cret. Foss. N.Am., 7. 



Shell very small, vertically ovate, being about one-sixth higher than 

 wide, narrowing upward from the middle ; base regularly rounded ; hinge- 

 line about half the transverse diameter of the valves ; ears unequal, posterior 

 one obliquely truncate, and hardly half as large as the other; anterior sub- 

 trigonal, flattened and slightly pointed. Right valve ornamented with com- 

 paratively strong, regular, raised concentric ridges and furrows. Left valve 

 smooth, or showing on exfoliated surfaces, traces of faint radiating striae ; ante- 

 rior margin with only a shallow, rounded sinuosity under the ears, nearly as 

 deep in the left as in the right valve. 



Length, 0.19 inch ; height, 0.23 inch. 



' I know of no fossil or recent species liable to be confounded with this 



little shell. It is perhaps more nearly like Pecten simplictis, Con. (Jour. 



Acad. Nat. Sci., IV (n. s.), pi. 46, fig. 44), than any other known form; but 



will be at once distinguished by the concentric ridges of its right valve, and 



the more regularly rounded outlii t' its lateral margins, as well as by the 



different form of its cars. 



