CAEBONIFEROUS SPECIES. 65 



rather narrow, distinctly incurved, so as to pass within the hinge-line; ears 

 small, nearly rectangular; lateral margins sometimes faintly sinuous near the 

 ears, and rounded to the front, which is rounded or a little sinuous in outline 

 at the middle, as seen from above; surface ornamented by rather broad, 

 regularly-arranged, concentric undulations, which become obsolete on the 

 umbonal region, and more strongly defined and wider anteriorly, while 

 on the umbonal portion there are numerous minute, slightly- elongated 

 prominences, arranged in quincunx, which probably bore minute spines; 

 farther forward the little spine-bases are arranged in concentric bands, 

 mainly in the furrows between the concentric undulations; spines appar- 

 ently all small, short, and inclined forward, or more or less depressed; interior 

 with tlie scars of the cardinal muscles narrow, nearly parallel, deeply 

 striated, and very profoundly impressed, so as to form on internal casts two 

 very prominent, parallel ridges, between which are seen the distinctly- 

 corrugated scars of the adductor muscles. Dorsal valve flattened, concave 

 over the whole visceral region, excepting a slight mesial ridge corresponding 

 to the sinus of the other valve, and marked by numerous small, regular, 

 concentric wrinkles, with many little projecting points that seem to have 

 borne small spines, especially toward the anterior and lateral margins. 

 Interior unknown. 



Length of one of the largest specimens, measuring over the curve of 

 the ventral valve from the beak to the front, 4.30 inches; breadth, 1.7G 

 inches; convexity, 1.30 inches. 



For about ten years past I have, at different times, had under consid- 

 eration specimens of this Produdus, without being able to identify it with 

 any of the described species. I have had no difficulty in separating it from 

 all of other known forms from the Far- West, and have always found it to be 

 quite constant in its characters. In young examples, it is much like F. 

 Nelrascenais of Owen;* though in adult specimens, it is found not only to 

 attain a much larger size, but to be proportionally greatly more produced 

 in front, as well as much more gibbous ; while it evidently did not possess 



* Not as represented by Dr. Oweu's tigure giveu under that name, on plate v of 

 Iiis Wisconsin, Iowa, and Minnesota Report, however, which figure certainly does not 

 represent the species described under that name by Dr. Owen, as may be seen by reading 

 his description, and as I know from examining his type-specimens. 

 5 P R 



