Trenton limestone. 
167 
216. 5. AMBONYCHIA OBTUSA ( n. sp.). 
Pl. XXXVI. Figs. S a, b. 
Obliquely ovate, short, gibbous ; umbones short, obtuse, scarcely incurved or bending 
forwards ; shell somewhat compressed towards the lower margin, convex on the centre and 
becoming inflated above ; anterior side obtuse, rounded, scarcely extending beyond the 
umbones; posterior side compressed, scarcely alated ; cardinal line straight, margin of the 
shell curving from its posterior extremity ; surface? 
The specimens seen are casts, where the markings of the shell are not preserved. This 
species is distinguished from the others by its short ovate form, as well as the shorter, very 
obtuse and gibbous umbones. It departs somewhat from the typical forms of the genus; 
but it has nevertheless the essential features, and cannot be referred to any other genus. 
I have a specimen from the northwest part of Wisconsin, which is apparently identical 
with the one here figured, but presents a slight ridge extending from the summit down the 
anterior side of the shell, as if produced by a contraction in that direction. The specimen 
is a cast, differing in no other respect from the one figured. 
Fig. 8 a. Right valve of this species, b. Profile view from the posterior side. 
Position and locality. In the higher part of the Trenton limestone, associated with the 
preceding, at Watertown. 
217. 6. AMBONYCHIA? 
Pl. XXXVI. Figs. 9 a, b. 
The specimen here figured has been unfortunately lost, so that a description cannot be 
given. It bears in some respects the characters of the present genus, but is equilateral, with 
a distinct prominent umbo. The surface is marked by thin sharp concentric ridges, as 
shown in the figure. 
Fig. 9 a , b. View of the single valve, and profile of the same. 
Position and locality. In the thinbedded higher portions of the rock at Middleville. 
There are several other species of the preceding, or allied, genera of Acepiiai.a, in the 
Trenton limestone ; but the condition of the specimens which I have seen is such that they 
cannot be satisfactorily described. Further examination will doubtless increase the number, 
and render us better acquainted with the character and relations of those already described. 
Notwithstanding that several of these species are widely distributed, and may be found 
in almost every locality of the rock, they are never abundant. Of several species^ only two 
