PLATE 17. 



Page 

 Fig. 1. Strophodonta perplana 98 



1 a. A young iDdividual. 



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 somenliat contracted on the liinge line. 



1 (. 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. 



If. The exterior of a ventral valve from the Hamilton group, at Kock Island, Illinois, show- 

 ing the unequal or fasciculate striae. 



1 g. The interior of the same, showing the mu-scular 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 /. A further enlargement of the hinge line, cardinal process, etc., ft-om another individual. 

 The cordiform extremities of the process show the striated surface for the attachment of 

 the muscles. 



1 »i. A longitudinal section. 



1 n. An enlargement of the surface strise of a New Tork specimen, showing the suh-equal strise. 



1 o. An enlargement from a Kock Island specimen, (1/,) showing the alternating or fascicular 



arrangement of strise. 



Fig. 2. Strophodonta demissa 101 



2 a. A young individual. 



2 4, c, d. Ventral valves of the ordinary form. 



2 e. A dorsal valve having strong sharp stria;. 



2/, g. Dorsal valves of the ordinary form, showing the area. 



2 k, I, 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 flg. 2 c, which is more than usually gibbous. 



2 p. An enlarged cardinal view, showing the areas of the two valves which arc destitute of a 



central callosity. 

 2r. The interior of a ventral valve, showing the muscular impressions. The area has been 



broken by the separation of the opposite valve. 

 2 ». The interior of a dorsal valve, showing the muscular impressions, the cardinal process and 



crenulated cardinal margin. 



Strophodonta bubdemissa 214 



2 A, i. Dorsal and ventral views of a specimen (Vom Rock Island, Illinois. 



