wars] LARAMIE FOSSILS. (7 
extreme thinness of the shell and its very elongated depressed form. 
The specimens here under consideration show that the thinness of the 
shell is not a constant character, though they at the same time show that 
this type presents other more important differences, of which I had seen 
indications before, but which I did not mention especially because the 
specimens then seen were not sufficiently well preserved to permit these 
characters to be clearly defined. They are differences in the hinge. 
For instance, although the primary teeth do not differ materially from 
those of Corbicula and Cyrena, the anterior lateral tooth differs from 
that of Cyrena in being linear and elongated parallel to the hinge-mar- 
gin, as well as slightly striated, thus agreeing with the corresponding 
tooth of Corbicula. Its posterior lateral tooth, however, on the other 
hand, is more nearly as in Cyrena, being shorter than in Corbicula, and 
placed very remote from the cardinal teeth, while the intervening car- 
dinal margins are wide, flat, and, when the valves are united, close fit- 
ting. Yet this tooth is also striated, as in Corbicula, though less dis- 
tinetly. Again, the ligament is also decidedly longer than in Corbicula, 
or than is usual in Cyrena, and also less prominent, there being appar- 
ently no elevated fulcrum for its attachment. The pallial line shows a 
shallow subsemicireular sinus. 
“¢‘T¢ will thus be seen that these shells combine some of the characters 
of both Cyrena and Corbicula, without agreeing exactly with either. It 
is well known to paleontologists, however, who have studied fossil shells 
of these groups, that there are many species that show intermediate 
characters between these genera, so that some eminent authorities do 
not admit the genus Corbicula, but place the whole under Cyrena. Should 
this view prevail, the forms here under consideration might be so dis- 
posed of. Still, even in that case it would seem desirable and conven- 
ient to separate them subgenerically by writing the name Cyrena (Lep- 
testhes) fracta. If the two groups, Cyrena and Corbicula, however, are 
to be regarded as distinct genera, there would be nearly or quite as good 
reasons for regarding Leptesthes as a genus.” 
In some remarks upon C. (Z.) planumbona in the Bulletin of this sur- 
vey, 2 series, No. 1, p. 45, Mr. Meek further notices the fact that the 
pedal muscular scar in this subgenus is detached from the anterior ad- 
ductor scar, and not blended with the latter. This characteristic is 
shown in figure 4 e, on plate 23, accompanying this article. For other 
characteristics of Leptesthes, not mentioned by Mr. Meek, see remarks on 
a following page, following the description of C. (Z.) macropistha. 
CORBICULA (LEPTESTHES) PLANUMBONA Meek. 
Plate 21, figs. 2 a, b, c, and d. 
Corbicula? (Leptesthes) planumbona Meek, 1875, Bull. U. 8. Geol. Sur. Terr., 2d ser., 
No. 1, p. 43. 
This species, so far as it is yet known, is confined to the Laramie 
strata east of the Rocky Mountains in Colorado, where, however, it 
appears to be comparatively abundant and to have a considerable 
geographical range. The type specimens from which Mr. Meek drew 
the following description were all imperfect, and the illustrations on 
plate 21 are therefore drawn from some fine examples obtained by my- 
self in the valley of Crow Creek, Colorado: 
“Shell attaining a moderately large size, rather thick and strong, 
especially about the hinge of large specimens, generally of a short, trans- 
versely oval or subelliptic form but rather variable in outline, moder- 
