PLATE 17. 
Page 
1. Strophodonta perplana. 98 
1 a. A young individual. 
1 b . An individual of medium size, having the cardinal angles extended. 
1 c. A large ventral valve, having a cicatrix along the median line—a feature of not uncommon 
occurrence in this species. 
1 d. A dorsal valve which is somewhat contracted on the-hinge line. 
1 e . An enlarged cardinal view, showing the areas of the two valves and a slight callosity on the 
dorsal area. The area is much wider than usual. 
1 f. The exterior of a ventral valve from the Hamilton group, at Rock Island, Illinois, show¬ 
ing the unequal or fasciculate striae. 
1 g. The interior of the same, showing the muscular impression. 
1 h. The interior of a ventral valve from the Hamilton group of New York. 
1 i. The cast of a ventral valve, showing more distinctly the large flabelliform divaricator mus¬ 
cular impressions and the small occlusor muscular impressions. 
1 k- An enlargement of the interior of a fragment of a dorsal valve, showing the peculiar form 
of the cardinal process. The continuation of the process along the centre, forming the 
median septum, is exfoliated. 
1 l. A further enlargement of the hinge line, cardinal process, etc., from another individual. 
The cordiform extremities of the process show the striated surface for the attachment of 
the muscles. 
1 m. A longitudinal section. 
1 n. An enlargement of the surface striae of a New York specimen, showing the sub-equal striae. 
1 o. An enlargement from a Rock Island specimen, (1 /,) showing the alternating or fascicular 
arrangement of striae. 
2. Strophodonta demissa. 101 
2 a. A young individual. 
2 b, c, d. Ventral valves of the ordinary form. 
2 e. A dorsal valve having strong sharp striae. 
2/, g. Dorsal valves of the ordinary form, showing the area. 
2 k, t, m. Ventral, dorsal and profile views of a large individual. 
2 n. Profile of a moderately convex specimen. 
2 o. A profile of the specimen fig. 2 c, which is more than usually gibbous. 
2 p. An enlarged cardinal view, showing the areas of the two valves which are destitute of a 
central callosity. 
2 i. The interior of a ventral valve, showing the muscular impressions. The area has been 
broken by the separation of the opposite valve. 
2 s. The interior of a dorsal valve, showing the muscular impressions, the cardinal process and 
crenulated cardinal margin. 
Strophodonta subdemissa. 214 
2 h, i. Dorsal and ventral views of a specimen from Rock Island, Illinois. 
