NATURAL SCIENCES OF PHILADELPHIA. II 



Modiolopsis subnasuta, M. and W. 



Shell rather small, elongate, narrow and slightly arcuate, rather distinctly 

 convex, the most gibbous part being along the posterior umbonal slopes, 

 above the middle ; dorsal and ventral margins slightly diverging posteriorly, 

 so as to make the widest (highest) part of the valves nearest the posterior end, 

 while the most sinuous part of the ventral margin is a little in advance of the 

 middle; anterior end narrow, a little produced, with an oblique forward 

 slope of its upper margin, to its narrowly rounded extremity ; posterior mar- 

 gin somewhat cuneate, with an oblique truncation more or less convex in 

 outline, to the posterior basal extremity, which is narrowly rounded ; cardi- 

 nal margin long and a little arched; beaks much depressed, and placed rather 

 nearer the anterior edge than to the middle ; posterior umbonal slope.-; promi- 

 nent, and forming an obtuse ridge, which extends obliquely backward to the 

 posterior basal edge of each valve ; anterior muscular scar comparatively 

 large, round, shallow, and placed near the edge of the valves ; small pedal 

 muscular scars distinct just above those of the anterior adductors; surface of 

 internal cast showing moderately distinct, irregular, concentric undulations, 

 which are most strongly defined below, and in front of the posterior umbonal 

 slopes, on the flattened or concave flanks. 



Length, 1-31 inch; height, 0-50 inch; convexity, 0-40 inch. 



This is a neat, symmetrical shell, resembling 31. nasuta, Conrad (sp.), but 

 differs in having the narrowed anterior end less produced, the beaks being 

 placed farther forward; while its posterior end is broader, and obliquely 

 truncated, instead of being rounded. Its general outljne is more like that of 

 Orthonota contractu, Hall (Palseont. N. Y., vol. i, pi. 32, fig. 8), though its lower 

 margin is less distinctly sinuous, its beaks more depressed, and its posterior 

 margin more oblique ; while it wants the oblique dorsal wrinkles seen on 

 casts of that shell, being a true Modiolopsis. 



Compared with foreign species, our shell is found to be very closely allied 

 to an English Upper Silurian species described by Mr. Salter under the name 

 M. platyphylla. Our shell, however, has the anterior end narrower and more 

 produced, with more prominent posterior umbonal ridges. 



Locality and position. Galena Limestone of the Lower Silurian, in Carroll 

 County, Illinois. 



SCHIZODUS AMPLUS. 



Shell attaining a large size, as determined from internal casts, moderately 

 convex, oblong-suboval in outline, or about one-fourth of its length longer 

 than high ; anterior side short, rounding from above the middle into the base : 

 outline of ventral margin forming a broad semi-ovate curve ; posterior margin 

 nearly vertically subtruncated, but rounding abruptly into the dorsal margin 

 above and into the base below ; cardinal border nearly straight, and sloping 

 very slightly from the beaks posteriorly ; beaks much depressed, nearly ante- 

 rior, incurved, closely approximated, and directed forward and inward. Pos- 

 terior adductor muscular scar shallow, rather large, suboval, and located close 

 up under the posterior extremity of the hinge ; anterior do., smaller, deeper, 

 subovate, and placed very near the anterior margin a little above the middle, 

 with rather distinct, nearly detached, pedal muscular scars at their upper 

 ends. Posterior umbonal slopes with each a distinct sulcus extending from 

 the beaks obliquely backward and downward, becoming wider and more 

 shallow as they descend, so as to die out before reaching the posterior basal 

 margin. (External surface unknown.) . 



Length of cast, 2-60 inches; height of do., 2-15 inches; convexity, 1-56 



111 This' is the largest species of Schizodus (if it belongs to that genus) we have 

 yet seen. Our only specimen is an internal cast, giving no idea of the nature 

 of the hinge. From its general appearance, however, and the nature of its 



1870.] 



