Tropidoi.ki'tus Fauna at Canandaku'a Lark, N. Y 



89 



Mi/sc/e Scars. — In young stages the muscle scars are so faint 

 that they are ahnost indistinguishable. The smallest ventral valve, in 

 which anything definite can be made out, is 4 x 4 mm. In it the 

 diductors have oval, somewhat widely separated, scars, between which 

 are the two small adductors, one on either side of the median line. 

 The diductors are bounded posteriorly by two ridges which make a 

 wide angle with each other. The adductor scars have faint ridges on 

 either side and another ridge between them. In a little later stage 

 these three ridges around the adductors become sharp and distinct. 

 The median one runs back nearly to the beak, while the outer ones 

 remain short, sharp and rather high, curving outward. In the later 

 neanic stages these ridges arch over and join the ridges which bound 

 the diductors (Fig. 6). The two ridges which bound the posterior 

 6 ' 7 8 9 



Fig. 6. Stropheodonia ina-quistriata (Conrad). Neutral muscle area and ridges 

 of an individual in later neanic or ephebic stages. X 2. 



Fic. 7. The same species. The corresponding area in ventral valve of a s]-)eci- 

 men in later ephebic or gerontic stages. X 2. 



Fli;. 8. The same species. Interior of the dorsal valve of a young specimen, 

 showing cardinal process and ridges in front of it. X 2. 



Fic,. 9. The same species. Interior of the dorsal valve of an adult, showing the 

 two pairs of adductor muscle scars and the brachial ridges in front of them. X 2. 



Figs. 6-9 are from specimens in the writer's collection. 



borders of the diductor impressions send off processes a short dis- 

 tance in front of the hinge which turn inward and run parallel 

 for a short distance-. They rise sharply from the floor of the valve 

 and overhang on the side toward the median line. In the later ne- 

 anic stages the whole extent of the diductor impressions is bordered 

 by a long sharp ridge which is later resorbed. 



During the adult stage, the parallel portions of the two ridges 

 which bound the diductors are extended and strengthened, and the 

 divergent portion is resorbed. The median ridge becomes stronger 

 and rounded, the two sharp ridges which separated the adductors from 

 the diductors disappear, and an almost square muscle scar is pro- 

 duced. In this scar the diductors are very plain, and the adductors 

 are narrow and situated somewhat posteriorly. Behind these is a 

 median pedicle muscle (Fig. 7). 



