240 



Annals of the Carnegie Museum. 



process, and finely striated surface. Type, PlcEsiomys strophome- 

 noides Raymond. Strophomena deflecta Conrad, S. recta Conrad, and 

 Orthis loricjila Hall and Clarke also belong here. This subgenus differs 

 from Flcrsiomys Hall and Clarke in ha\ing the surface much more 

 finely striate, and in retaining the deltidium throughout life. 



23. Valcourea strophomenoides Raymond. 



(Plate XXXV, figures 15-19; plate XXXVI, figure i.) 

 Plasiomys strophomenoides Raymond, 1905. American Journal of Science, series 

 4, Vol. XX, p. 370. 



Description. 



Shell small, pedicle valve convex at the umbo, concave in front. Brach- 

 ial valve convex, with a narrow sinus on the umbo. Surface marked 

 by numerous fine striae which increase by bifurcation and implanation. 

 There are usually seven or eight in a space of two millimeters on the 

 front of the shell. The cardinal area of both valves is low. On the 

 pedicle valve there is a convex deltidium, which 

 is perforated at the apex for the passage of the 

 pedicle. The interior of the pedicle valve shows 

 a small but strongly impressed muscle scar under 

 the umbo. The scar is roughly quadrate, and 

 contains a pair of strong diductor scars, between 

 which are the scars of the adductor muscles. 

 Behind the latter is a deep pedicle scar. The 

 lateral edges of the diductor scars are bounded 

 by strong plates which run back to support the 

 teeth. The interior of the brachial valve shows 

 a robust, simple cardinal process and small dental sockets, bordered 

 by strong plates which do not diverge greatly. In front of the car- 

 dinal process is a low median septum, on either side of which are the 

 four scars of the adductors, not, however, deeply impressed. 



This species can be distinguished from Plasiomys platys by its much 

 finer striae and the much more convex brachial valve. It is com- 

 moner than the former species, and is usually found in the middle 

 of the Chazy. 



Locality. — Crown Point, Valcour Island, and Plattsburgh, New 

 York. The cotypes are from near the Fair Grounds at Plattsburgh. 

 and are in the Carnegie Museum.' 



Fig. 12. Valcourea 

 strophomenoides Ray- 

 mond. A small 

 specimen, to show 

 surface markings. 

 X 2. 



