Fossils oj the Niagara and Clinton Grotips. 135 



Genus Ortiiis, (Dalman.) 



The shells of the Genus Orthis are usually small, few of them exceeding 

 one inch in diameter ; they are generally nearly circular or quadrate, the 

 hinge line is straight, and in most of the species shorter than the width of 

 the shell. The valves are either equally or unequally convex, the ventral 

 valve is often the longest, the beaks are more or less incurved, that of the 

 ventral valve generally most prominent. The surface usually striated or 

 ornamented by ridges radiating from the beak to the margin. Both valves 

 have an area, and the foramen is partly excavated in both. The foramen of 

 the dorsal valve is partly filled by a small cardinal tooth-like process, from 

 which a small rounded ridge proceeds along the interior surface of the shell 

 with two muscular impressions on each side, placed obliquely one above the 

 other, (see fig. 6, page 130.) The muscular impressions in the ventral 

 valve consist of two elongated depressions beneath the beak, usually divided 

 by a small mesial ridge, (see fig. 7, page 130.) From each side of the 

 foramen in the dorsal valve, two small, slender processes project, to 

 which were, no doubt, fastened the free fleshy spiral arms. Many of the 

 gpecies have also a small tooth on each side of the foramen of the ventral 

 valve. 



The genus commenced to exist in the Lower Silurian epoch and con- 

 tinued until the carboniferous period, above which no specimen have been 

 found. 



Genus Strophomena, (Rafinesque.) 



In this genus the shells have a very straight hinge line which is generally 

 as wide or wider than the body of the specimens. They are semi-circular, 

 cemioval, or quadrate in form. One valve is convex, and the other concave 

 on the outside. The two valves curve into each other ; sometimes it is the 

 ventral and sometimes the dorsal, which is concave. The area occupies both 

 valves ; it is largest, and partly covered by a thin shelly growth called the 

 ' * deltidium," in the ventral valve. The beak of the ventral valve is either 

 entire or perforated by a small circular aperture. The foramen in the dorsal 

 valve is also partly occupied by a cardinal boss or process. The muscular 

 impressions in the dorsal valve are not situated one above the other as in 

 Orthis, but beside each other in a direction across the valve. Those in tba 

 ventral valve occupy a saucer-shaped cavity near the beak. 



The genus appeared in the Lower Silurian and continued into the 

 carboniferous epoch. 



Genus Leptena, (Dalman.) 



The same as Strophomena, except that in the ventral valve the muscular 

 impressions are not bordered by a ridge forming a saucer-shaped cavity, 

 while in the dorsal valve they are large and long, extending from near the 

 beak downwards two-thirds of the length of the shell. 



The genus commenced in the Lower Silui'ian and continued to the lattw 

 part of the Lias period. 



