CAUBOXIFKKOUS AGE. 35 



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

 It also r< -t 'lulilcs ('<>i-alli<,j,/ t <i<j,i, from which, however, it is clearly distinguished by 

 its simple pallial line. 



Prof. M'('o\. \N!IO lias had an opportunity to compare t\pical specimens of 



f '/ i,li,j,/tni-nt<, with examples of J'/, n, i,],/,,, ,</.< showing the hinge, sa\- 1 1 icy agree 



tly 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 /'// n,-"/>/t<>rit.-<. 



Prof. Hall's name, lioui M r, ma\. we think, be properly retained in a subgcneric 

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

 subgenus may lw distinguished from the t \pical costated species found in the more 

 modem formations, by the following characters: 



HALL. 

 Shell without radiating pottero-donal cost*. Internal ridge bounding the tear of the anterior adductor, 



descending with a slight forward slope. 

 Type. \aculitfi planulata, Cos BAD. 



The genus Pfenrop&ontf, 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. 



PJcurophortiK occidental!*. 



(PLAT! I, Fig. 11, a, i.) 



Pleuroplionut oeeidentalti, MEEK & HAYDKX, Trans. Albany Institute, IV, March 2, 185. 



Shell small, narrow, somewhat elongate; valvei moderately convex along the nmbonal sloped. Basal and 

 dorsal margins nearly straight and snbparallel, or converging slightly toward the front ; posterior tide rounded, 

 and rather compressed ; anterior side rouuding np 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 fire small faintly defined radiating costae, extending from tbo 

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



Length, 0.37 inch ; height, O.lfi 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 really 

 belongs to the typical group J'l< tini/i/Kiru*. It agrees, however, so nearly in its 

 external characters, with P. costattw, 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. coxtadw, in being much 

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



Locality and jxmition. Nebraska, nearly opposite the northern boundary of 

 Missouri. Coal Measures. (No. 1017.) 



