47 



front, which is generally more or less produced, and, as seen in a direct 

 view from above or below, transversely truncated or a little sinuous at 

 the middle. Dorsal valve very convex, particularly along the middle, 

 the elevation increasing rapidly to the front, which is raised so as to 

 form a very prominent, broad, rounded, or somewhat flattened, and 

 slightly-defined mesial fold, rarely traceable back to the ceutral region,, 

 while, on each side, the lateral slopes descend abruptly to coaaect with 

 those of the other valve ; beak moderately prominent, and incurved mOre 

 or less nearly at right angles to general plane of the valves ; interior 

 with a prominent mesial septum extending forward nearly half-way to 

 the front. Ventral valve flattened at the umbo, and so broadly and pro- 

 foundly sinuous from near the same anteriorly as to leave only a promi- 

 nent angular margin on each side, the sinus bemg broadly flattened along 

 the middle, and increasing rapidly in depth to the front margin, which 

 is curved upward more or less nearly at right angles to the plane of the 

 valves, and produced in the middle, in the form of a large extension 

 fitting into a corresponding sinuosity in the middle of the front of the 

 other valve ; anterior lateral margins on each side of the sinus meeting 

 those of the other valve at acute angles ; posterior lateral margins very 

 abruptly deflected and rectangularly deflected along each side of the 

 sinus, to meet those of the other valve; beak comparatively small. 

 Surface of both valves ornamented by numerous radiating costse, which, 

 on the umboues, are merely distinct raised lines, but increase in size 

 anteriorly, particularly those in the sinus and on the mesial fold, where^ 

 toward the front of adult si)ecimens, they become moderate-sized, 

 rounded ribs, of which four to six or seven may be counted in the imme- 

 diate flattened bottom of the sinus, and two or three more on the fold, 

 while those on the lateral slopes bifurcate, and continue, of smaller size, 

 to the anterior and antero-lateral margins. (Finer surface-markings 

 unknown.) 



Length of an adult specimen, 1.78 inches; breadth, 1.53 inches y 

 convexity, about 1.24 inches. 



This is a fine species, more nearly resembling some Devonian and 

 Upper Silurian forms than the usual Carboniferous types. Its most 

 marked features are the large size of its mesial sinus, the flattening of 

 its posterior lateral slopes, and the angularity of the posterior lateral 

 margins of its ventral valve on each side of the sinus, formed by the 

 abrupt flexure of those margins to meet those of the other valve. This 

 inflection of the posterior lateral margins gives this part of the shell a 

 peculiar truncated, rectangular appearance, contrasting birougly with 

 the 'very acute angles formed by the connection of the antero- lateral 

 margins of the valves. 



The specific name is given in honor of Dr. Endlich, of the United 

 States geological survey of the Territories. 



Locality and position.— Ea^st of Animas Eiver, Colorado Territory, 

 where it occurs associated with a small Froductus of the type of P. sub- 

 acideatus. ^ According to Dr. Endlich's sections, as well as from its 

 affinities, it would seem to be most probably an Uoper Devonian species. 

 Fragments of it have been brought in from other localities in the Rocky 

 Mountains. 



