Iviii SYNOPSIS OF GENERA. 



Considerable confusion has arisen as to the validity and chariicters of the 

 genera Pemopecten and Entolium, and a short historical sketch, and critical dis- 

 cussion of these genera is here given. 



1865. In the Report of the Geological Survey of California, vol. 1, Apjjen- 

 dix B, p. 478, Mr. Meek proposed the name Entolium as a subgenus of Amus- 

 vmts of Klein, for a species (E. aurarium) found in the auriferous slates of 

 California, and supposed to be of Jurassic age (pp. 477-479 ut cit.). The figure 

 illustrating this species is of a pectenoid shell, the external surface of which 

 presents the concentric striae without radii ; the internal characters not shown 

 nor described. 



At the end of the description of Amussium aurarium Mr. Meek says : " This 

 species appears to belong to a group of thin lenticular, sub-equivalve, smooth 

 or concentrically marked shells, with short, equal, flat, obtusely angular ears, 

 which are not defined in either valve, or on either side, by a distinct marginal 

 byssal sinus. They differ from the typical species of Amussium, in having the 

 lateral margins closed, and the valves without internal radiating costae. This 

 group may be distinguished by the name Entolium, with Pecten demissus, of Phil- 

 lips (as figured by Quenstedt, in his Der Jura, i, tab. 48, f. 6 and 7), as its type." 



1872. In the Final Report of the U. S. Geological Survey of Nebraska, 

 p. 189, Mr. Meek cites the genus Entolium and publishes under it E. avicula- 

 tum. Swallow sp.^Pec/en aviculatus. Swallow (Trans. St. Louis Acad. Sci., vol. 1, 

 p. 215. 1858). After describing the shell, Mr. Meek says: "This shell evi- 

 dently belongs to a group for which I proposed, in the California Report, the 

 name Entolium with Pecten demissus, Phillips (not Fleming), as illustrated in 

 Quenstedt's Der Jura, pi. 48, fig. 6, as the type." Mr. Meek illustrates E. avi- 

 culalum, pi. 9, fig. 11, and also copies from Quenstedt the hinge structure of 

 E. demissum, in illustration of the generic characters (ut cit., pi. 9, fig. 14). 



1873. In the Report of the Geological Survey of Illinois, vol. V, p. 588, we 

 find the following : Genus Entolium, Meek. Entolium aviculatum. Swallow sp., 

 pi. 26, figs. 12o, c. 



Pecten aviculatus, Swallow, 1858. Trans. St. Louis Acad. Sci., vol. 1, p. 213. 

 Entolium aviculatum, Meek. Final Report on Neb., p. 189, pi. 9, figs. 11a, /. 



