lviii 
SYNOPSIS OF GENERA. 
Considerable confusion has arisen as to the validity and characters of the 
genera Pernopeden 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, Appen¬ 
dix B, p. 478, Mr. Meek proposed the name Entolium as a subgenus of Amus- 
uims 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 striee 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 Peden 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 ,=Peden 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 Peden demissus, Phillips (not Fleming), as illustrated in 
Quenstedt’s Der Jura, pi. 48, fig. 6, as the type.” Mr. Meek illustrates E. avi- 
culatum, 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. 12a, c. 
Peden 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, /. 
