CARBONIFEROUS AGE. 35 



in having a short niDper cardinal tooth ; and in being destitute of anterior teeth. 

 It also resembles CoraUiojiJiaga, from which, however, it is clearly distinguished by 

 its simple pallial line. 



Prof. McCoy, who has had an opportunity to compare typical specimens of 

 CleidopJiorus, with examples of Pleuroj^Jwi-us showing the hinge, says they agree 

 exactly in their dentition, and it is on his authority that we here regard them as 

 synonymous. Our description of the hinge is taken from Prof. King's carefully 

 written description of Pleiiroj^Jiorus. 



Prof. Hall's name, however, may, we think, be properly retained in a subgeneric 

 sense, for a section of this group probably confined to the Silurian rocks. This 

 subgenus may be distinguished from the typical costated species found in the more 

 modern formations, by the following characters : — 



Cleidopliorus, Hall. 



Shell without radiating postero-dorsal costse. Internal ridge bounding the scar of the anterior adductor, 



descending with a slight forward slope. 

 Type. — Nuculites planulataj Conrad. 



The genus Pleuroj^Jiorus, as here defined, was introduced during the Lower 

 Silurian epoch, where it was represented by the smooth species. It also ranges 

 through the Carboniferous and Permian rocks, as stated above, into the Trias, the 

 more recent species being the typical forms. 



PJeurophorus occidentalis. 



(Plate I, Pig. 11, a, b.) 

 Pleurophorus ? occidentalis, Meek & Hatden, Trans. Albany Institute, IV, March 2, 185. 



Shell small, narrow, somewhat elongate ; valves moderately convex along the umbonal slopes. Basal and 

 dorsal margins nearly straight and subparallel, or converging slightly toward the front ; posterior side rounded, 

 and rather compressed ; anterior side rounding up gradually from the base. Beaks small, depressed, and located 

 at the anterior extremity ; hinge line long and straight. Surface with a few concentric marks, and traces of finer 

 lines of growth, which are crossed by some four or five small faintly defined radiating costse, extending from tho 

 beaks obliquely backwards and downwards to the posterior, and postero-basal margins. 



Length, 0.37 inch ; height, 0.16 inch ; breadth, or convexity of the two valves, about 0.14 inch. 



Not having seen the hinge or interior of this little shell, we are not sure it reaUy 

 belongs to the typical group Pleuroplwrus. It agrees, however, so nearly in its 

 external characters, with P. costatus, as figured by Prof. King in his work on the 

 Permian fossils of England, that we are^ strongly inclined to think it will be found 

 to possess a similar hinge. Specifically, it differs from P. costatus, in being much 

 smaller, more depressed, and more contracted in the antero-ventral region. 



Locality and position. — Nebraska, nearly opposite the northern boundary of 

 Missouri. Coal Measures. (No. 1017.) 



