Tropidolei'tus Fauxa at Caxandakuja Lake, N. Y. 



99 



Fig. 1 6. Stropheodonta 



muscle area to show the 

 two pairs of adductor 

 scars (the small triangles 

 without arborescent mark- 

 ings represent the ante- 

 riors) the low median sep- 

 tum ending in a sort of 

 saddle, and the pustulose 

 ridges, not muscle scars, 



scars are divided into two elements, the posterior adductors, which 

 are large and oval, and in front of and between these, the small tri- 

 angular anteriors. In front of the muscle scars 

 there is a median septum and on each side of 

 it a rounded postulose ridge. Outside these 

 ridges are two lower, sharper ridges one of which 

 starts in front of each anterior muscle scar, curves 

 slightly outward, and then in again, but does not 

 approach much nearer the septum than it was at 

 the starting point. These ridges probably have concava \{z\\. Sketch of 

 to do with the brachial apparatus rather than with '''^ cardinal process and 

 the muscular system, as has been previously sup- 

 posed. 



S. junia Hall. 



Hall, Pal. N. V., IV, 1S67, p. 108, pi. i8, figs. 3, 4. 



Only one well preserved specimen of this spe- 

 cies was found in the collection. It is a ventral 

 valve, 44 mm. in length and 50 mm. wide. The 

 hinge is crenulated for three-fifths of the distance on either side of it. Nat- 

 from the beak to the cardinal extremities. The "'"^^ ^'^^■ 

 area about the beak is broken, so nothing could be learned about the 



deltidium or pedicle opening. The 

 scars of the muscles are very well 

 marked. The diductors are large, 

 flabelliform, faintly marked ante- 

 riorly, but with strong posterior 

 ridges extending from the area half 

 way down the sides. Between the 

 diductors are the adductor impres- 

 sions which are divided on the me- 

 dian line by a low, thin septum. 

 These scars are long, narrow and 

 acuminate in front. Two diagonal 

 lines divide the scars into anterior 



Fig. 17. Slropheodonta junia Hall ; , . , , , 



, , ,, , , ■ , , and posterior elements and the 



part 01 the muscle area of a ventral valve, ^ 



showing the anterior and posterior ele- posteriors have arborescent mark- 



ments of the adductors, and the pedicle ings, while the anteriors, which are 



muscle scar. X 3- smaller, do not. Back of these 



scars are two low processes, the remnants of the dental plates, and 



