296 



subequal, separated from the body of tlie shell by shallow constric- 

 tions. Beaks minute, not elevated above the hinge. Anterior lateral 

 margins straight to near the middle of the shell ; basal margins broadly 

 rounded; posterior straight, longer and more oblique to the hinge 

 than the anterior. Lateral borders flattened, outside of a line pass- 

 ing from the beaks to the extremities of the basal line, the entire 

 convexity of the shell being confined to the body of the shell, \7ithin 

 these lines, which gives to the body of the valve an elongate triangu- 

 lar form. 



Surface marked by fine, closely arranged concentric strise ; entirely 

 destitute of radiating strise. 



This shell differs from Avicula circulus, Shum., from the oolitic beds 

 of the same locality, in being more elongate, in the less extension of 

 the hinge line, in the flattening of the lateral spaces, and in being 

 destitute of the fine radiating strias of that species. It closely resem- 

 bles Pecten dissimilis, Flem., as figured by De Koninck, Anim. Foss., 

 pi. 4, fig. 7, in general form and proportions, but that species is desti- 

 tute of the peculiar flattening of the lateral borders, and is also 

 radiately striate. This species is also found in the Chemung of Ohio. 



AVICULO-PECTEN NODOCOSTATUS (n. sp.) 



Shell of medium size, semi-circular in outline, valves depressed 

 convex, the left one most ventricose. Hinge line straight, equalling 

 the greatest width of the shell. Anterior extension separated from 

 the body of the shell by a deep marginal sinus, and by a broad flat- 

 tened depression on the surface, extending from the beak to the 

 extremity of the auricle; posterior side having no sinus. Beak of 

 the right valve minute, depressed, that of the left ventricose, promi- 

 nent, situated at two-fifths of the length of the hinge from the ante- 

 rior extremity. 



Surface marked by from forty-five to fifty rugose, radiating plica- 

 tions, which sometimes bifurcate ; those on the body of the shell about 

 twice as wide as the interspaces ; while those of the sides are much 

 finer. The depression, separating the anterior auricle on the right 

 valve, has but one plication, that of the left valve marked by several. 

 Strong, undulating concentric lines cross the radii, giving them their 

 rugose surface. 



The semi-circular form, and the nodose character of the radii, will 

 serve to distinguish this from any other described from rocks of the 

 same age. 



Genus Mytilus, Linn. 



Mytilus fibristriatus (n. sp.) 

 Shell very elongate ovate, alternate at the beaks, more ventricose 

 below, extremely compressed toward the extremity of the hinge and 



