NATURAL SCIENCES OF PHILADELPHIA. 
51 
ward, but often curving gently outwards just below its junction with the ex- 
tremity of the hinge line, so as to make the angle of the small, more or less 
compressed posterior wing not more than eighty or ninety degrees ; hinge line 
less than the greatest length of the shell. Beaks scarcely oblique, located 
nearer the anterior than the posterior side ; that of the left or larger valve more 
prominent than the other, and extending a little above the hinge ; beak of the 
smaller valve more compressed, and scarcely distinct from the hinge margin. 
Surface marked by small, obscure, slightly elevated, radiating lines or cost?e, 
which are less than the spaces between, and crossed by fine, nearly obsolete 
lines of growth, usually most distinct near the upper anterior margin. Length 
'60 inch ; height '64 inch ; breadth '26 inch. 
The sinus for the passage of the pedal muscle in the anterior margin of the 
smaller valve is rather deep, narrow, and connected with a deeply impres- 
sed narrow groove, which extends on the outside of the valve nearly parallel to 
the hinge, quite to the beak. 
The surface markings are usually rather obscure on both valves, and often 
nearly obsolete on the smaller one. Young individuals are more nearly orbicu- 
lar than mature specimens. 
A closely allied representative of the Liassic species Monotis substriata Miinst. 
(see Leonh. Br. t. ii, p. 8, 406.) 
Locality and position. — Southwestern and eastern sides of Black Hills, ranges 
from upper part bed A, low down in bed C, but not in to B. 
Mytilus pertenuis. 
Shell small, extremely thin and fragile, slightly arcuate ; valves convex 
along the middle from near the beaks, obliquely backwards and downwards to 
the lower part of the posterior end ; extremities narrowly rounded, the anal 
end being a little broader than the other, and having its most prominent part 
below the middle. Base somewhat arched behind the middle, more prominent 
and curving very gradually upwards toward the front ; dorsum carinate from a 
little behind the beaks posteriorly, its outline forming a broad, sloping curve. 
Hinge nearly straight, rather short, rounding gradually into the dorsal edge 
behind. Beaks small, rather obtuse, subangular above, and located at the 
anterior end, scarcely projecting beyond the hinge and anterior margin. Sur- 
face marked by fine, rather obscure lines of growth. 
Locality and position. — West base of Black Hills, bed C, of the accompanying 
section. 
ArCA (Cl?CULLiEA) INORNATA. 
Shell oblong-oval, subrhombic, rather gibbous in the umbonial region ; an- 
terior side rounded up from below, so as to meet the hinge at an angle of about 
ninety degrees ; posterior side a little broader than the other, obliquely trun- 
cate above, somewhat narrowly rounded below ; base nearly straight along the 
middle, but not exactly parallel to the hinge, rounding up more gradually to- 
wards the front than behind. Beaks rising somewhat above the hinge, rather 
pointed, incurved and very slightly oblique, located a little in advance of the 
middle ; posterior umbonial slopes subangular. Hinge rather long, but not 
equalling the greatest length of the shell ; posterior teeth two or three in each 
valve, linear and elongate parallel to the hinge margin ; anterior teeth much 
shorter and oblique ; ligament area not very broad. Surface apparently smooth. 
Length "75 inch ; height '45 ; breadth -46 inch. 
Has the teeth of the hinge arranged like those of CucuUaa, or approaching 
those of 3facr odon, but the posterior muscular impression seems not to be 
raised upon a projecting lamina as in those genera. 
Locality and position. — South western base of Black Hills ; also around Beai 
Butte, on east side of Black Hills, bed A, of the foregoing section. 
1858.] 
