90 Anxals of jhe Carne(;ik Museum. 



In the brachial valve of some specimens the adductor muscle scars 

 are well defined and in others they are very faint. They are usually 

 bounded by a low ridge. The outer pair, the posterior adductors, 

 make up most of the scar. They are small, roughly triangular, and 

 situated close to the front of the cardinal process. The anteriors are 

 narrow, and are situated on a platform between and slightly above the 

 level of the posteriors (Fig. 9). 



Between the scars, on the median line, is a low, short septum 

 which is hardly elevated above the surface of the shell in young speci- 

 mens but becomes prominent in adults. On either side of it is a low 

 ridge which extends back nearly to the l)ase of the cardinal process. 

 In the adult the ])ortion of the ridges in front of the muscle scars 

 becomes high and incurved, and may function as a support for the 

 brachia. These ridges are short, and their anterior ends are not half 

 way to the front of the valve (Figs. 8 and 9). 



'i'hey appear to be homologous with similar ridges in Chouetes sci- 

 fiilits, which certainly are connected with the brachia. The structure 

 of the muscle scars and ridges in this species should be compared with 

 that of the dorsal valve of ^. concava (Fig. 16). 



Hinge Stnicfure. — The young specimens do not preserve the hinge 

 structure, but in adult and senile stages, the deltidium is present and 

 covers the deltyrium entirely. The pedicle opening is forced back 

 into the beak. In a specimen 7 mm. long and 11.8 mm. wide the 

 deltidium is continued into a short, exsert pedicle tube. The aper- 

 ture at the apex of this tube is about .05 mm. in diameter, which is 

 fully twice the diameter in some of the adult and gerontic specimens 

 examined. 



Crcniilaiious. — In the smallest specimen (3 mm. wide) uhich 

 showed any trace of crenulations, .66 mm. on each side of the 

 middle ])oint of the area was marked by faint depressions, about 6 

 on each side. Thus about .44 of the area was crenulated. A speci- 

 men 4.7 mm. wide had .43 of its hinge width crenulated. A spec- 

 imen 6 mm. wide has .66 of its width crenulated. Another specimen, 

 12 mm. wide, had .71 of its width crenulated. A specimen 24 mm. 

 on the hinge line, had crenulations which extended .56 of the dis- 

 tance between the cardinal extremities. A senile individual had 

 crenulations covering .57 of the hinge width. Thus it is seen that a 

 greater proportion of the hinge is crenulated during the later neanic 

 and adult stages than during the early adolescent or senile i)eriods. 



