86 



Annals of thk Carnegik Museum. 



width is about midway between the anterior and posterior margins. 

 In older shells the interior retains this oval form, while the beaks of 

 both valves project posteriorly, giving the exterior of the shell a 

 roughly triangular ai)pearance. The ventral beak is often more ex- 

 tended than that of the dorsal valve. 



The index of the nepionic shellis from 1.08 to 1.16. Of the neanic 

 shells, from .96 to .65. Among the adults there are both wide and 

 narrow forms. In the narrow ones, as seen from the inside, the 

 periphery is elongate oval, while in the wide forms, it is circular. In 

 general the index rises as the length increases, coming up from .76 in 

 a shell 2 mm. long to .90 in one 7.8 mm. long. In no stage is there 

 any evidence of a pedicle opening. 



Muscle Scars. — The muscular areas are shield shaped, with the 

 points directed forwards. The broad posterior side is close to the 

 posterior margins of the shell, and the anterior point reaches about 

 midway to the front. 



Fig. 2. PJtolidops. ohlata Hall. Ventral valve, showing anterior and posterior 

 unpaired scars, anterior and posterior paired scars, and median scar. X 4- 



Fig. 3. The same species. Dorsal valve, showing the corresponding muscles. 



X4. 



Both figures from specimens in the writer's collection. 



Fig. 4. Treiiiatis 7>iinep2t7icta Hall. Interior of dorsal valve after Hall and 

 Clarke for comjiarison with Fig. 3. Copied from Fig. 4. PI. 1\' Ci, Pal. N. V., 

 Vol. 8, pt. I. 



In the ventral valve, there is a narrow transverse scar forming the 

 front of the shield. In some specimens this seems to be made up of 

 two scars, one on either side of the median line of the shell. Behind 

 this are two large oval scars, situated obliquely, with their anterior 

 ends nearly, but not quite, meeting. Behind these, at the posterior 

 corners of the shield, are two smaller scars which are roughly trian- 

 gular in some specimens and nearly round in others. Between these 

 is an elongate scar, divided into two parts along the median line. On 



