ONONDAGA-SALT GROUP. 343 



ACEPHALA. 

 Genus MEG ALOMUS. 



Shell equivalved, longitudinal; umbones at the anterior extremity, abruptly incurved or 

 acuminate ; shell thick, anterior extremity extremely incrassated, and along the hinge-line 

 thickened interiorly nearly across the cavity, and in the left valve showing several shallow 

 folds or indentations ; muscular impression large and strong, extending deeply into the shell, 

 with two smaller circular pits just above. 



The general character of this shell is like that of Megalodok, though there are important 

 differences. In Megalomus, there is no depression in the thickened interior portion of the shell 

 (of the left valve) above the muscular impression ; nor is the elevated tooth-like projection 

 below and anterior to this depression, in Megalodon, present in Megalomiis, the similarity 

 in this part being simply in the folds on the posterior edge. The character of the muscular im- 

 pression is similar, though in Megalodon it is less deep and more longitudinal than in our 

 shell. 



707. 1. MEGALOMUS CANADENSIS {n. sp.). 



PL. LXXX. Fig. 1 a-e; Pl. LXXXI. Fig. 1 a-f; and Pl. LXXXII. Fig. 1 a-i. 



Shell ovoid, sub-cylindrical, the depth of both valves being equal to the height ; beaks an- 

 terior, incurved, a distinct groove along the inner edge of the dorsal margin from beneath the 

 beak two-thirds the distance to the posterior extremity ; shell extremely thick on the anterior 

 portion, composed of numerous lamin;e, which, when seen vertically, have an apparent fibrous 

 structure ; muscular impression very deep, the sides striated ; two smaller and shallower pits 

 above the larger one. Exterior surface of shell concentrically striated. 



The cast of this species, when not distorted, is very prominent at the beaks and along the 

 dorsal side, gradually sloping off to the posterior end. Below the umbones, and for some dis- 

 tance backwards, it is abruptly contracted or depressed from the thickening of the shell at this 

 part ; while the lower portion, with the cast of the muscular impression, projects forward as 

 far as, or a little in advance of the umbones. Between the muscular impression and the umbones, 

 there usually remain several folds with corresponding Indentations, marking the impressions of 

 the teeth or plications of the hinge. From the muscular impression, a shallow groove extends 

 backwards parallel to the margin of the shell, till it gradually dies out on the posterior half of 

 the shell. 



This fossil is found, for the most part, in the condition of casts, showing a great variety of 

 form and proportions, which seem all to be the result of distortion from pressure or other 

 causes. After examining a great number of these casts, I am unable to find any characters in- 

 dicating more than a single species. The comparatively few specimens retaining the shell ex- 

 hibit less distortion, and generally the proportions are well preserved. 



