6 CARBONIFEROUS LAMELLIBRANCHIATA. 



Generic Ch(irai-t(>r.<. — Shell compressed, somewhat oblique, subtriangularly trans- 

 verse, very inequilateral. Anterior end very small, sharp, and pointed; inferior 

 border becoming more and more convex as it passes downwards and backwards ; 

 posterior border sinuous. Hinge-line the longest part of the valve, straight; 

 umbones small, placed anteriorly but not terminal ; valves obliquely swollen from 

 the limbo towards the postero-inf erior angle ; posterior slope compressed, expanded, 

 and subalate. 



Interior unknown. 



Exterior surface ornamented with concentric lines and folds, more apparent 

 posteriorly and over the swollen part of the valve. 



Observations. — The genus Pteroiiites, M'Coy, contains shells of two very distinct 

 types, those with concentric and those with radiating ribs and strise. The first 

 species described by M'Coy under the generic heading Pferonites is P. angustatus, 

 and this must therefore be regarded as the generic type ; and P. kdus, M'Coy, 

 doubtless belongs to the same genus. The shells with radiating strige, P. snlcatus, 

 P. iiersulcatiis, differ not only in ornament but in shape, being much more quadrate, 

 having the posterior border less emarginate, the hinge-line comparatively much 

 shorter, the dorsal slope much more compressed, and the anterior end smaller but 

 more gibbose ; the umbones were nearly terminal. 



These characters are present in the Actinopteria , Hall, a genus well represented 

 in the Devonian rocks of New York State, and I now refer these species to this 

 genus. The genus Pferonites was adopted by Hall in this restricted sense, and he 

 gives the following diagnosis : — " Pteronites, M'Coy. This genus is restricted to 

 those species possessing the characters of the original types. Body very oblique ; 

 hinge-line longer than the body of the shell ; wing and hinge extended posteriorly. 

 Test marked by concentric stride." 



I have little or no hesitation in considering Arhndopiiinn of Meek as a synonym 

 of Ptrroiritrs. He founded this genus for Pinniform shells with non-terminal 

 umbones and a sinuous posterior border, both of which characters are well shown 

 in his type A. americana. I am uncertain as to the correctness of de Koninck's 

 reference of Pinna spatula, M'Coy, and other allied shells, to AriculopiniHi. I have 

 not yet been able to recognise in British specimens the characteristic features on 

 which de Koninck relies for his change of genus. 



Pteronitcs would therefore form a link between Pinna and Leiopteria, possessing- 

 a more Pinniform outline, but approaching the latter genus in the sulcation of the 

 posterior border and long extended hinge-line. 



