222 Annals of the Carnegie Museum. 



Ontogeny. 

 The nepionic shell is bi-convex, smooth, without fold or sinus. 

 It usually reaches a length of about i mm. The first change is the 

 introduction of a sharp median sinus in the dorsal valve, and a small 

 fold on the pedicle valve. Thus the fold and sinus at this stage have 

 a position just the reverse of that in the adult. Sharp plications are 

 now introduced on both valves, and new ones added at the sides so 

 rapidly, that, at a length of about 2 mm. the maximum number of 

 plications is attained. At this stage the great variation in the number 

 of plications can be observed. Some of these small shells have as many 

 as thirty plications on each valve, while others have only half that 

 number. In later stages of growth no new plications are added, except 

 in the very rare instances where one or more of the plications bifur- 

 cate. The dorsal sinus persists, though becoming shallower and wider 

 until the shell has reached a length of about 4 mm. When it reaches 

 a length of 7 or 8 mm. the ventral sinus and dorsal fold are formed. 

 The permanent ventral sinus occupies about half the front of the shell, 

 and takes in a variable number of plications — from four to seven — 

 according to the multiplicate or pauciplicate character of the shell 

 at this stage. 



Description of the Adult. 



The adult shells are sub-triangular to sub-circular in outline, with 

 a wide, shallow ventral sinus and a somewhat elevated dorsal fold 

 Surface marked by from seventeen to twenty-four strong plications, 

 four to seven of which are in the sinus and five to eight on the fold. 

 The plications are crossed by zig-zag lines of growth, which are some- 

 times stronger and sometimes weaker on partially exfoliated speci- 

 mens than on specimens with perfect shells. The dorsal beak is 

 strongly incurved, and the umbo bears a slight median depression. 

 The beak of the pedicle valve is only slightly incurved, and does not 

 rest against the brachial valve. The delthyrium is open throughout 

 life. None of the specimens in the collection show the deltidial 

 plates. 



Casts of the interior of the brachial valve show a low septum which 

 extends about one-third the length of the shell. This septum divides 

 at its posterior end as in the typical species of CamarotoscJiia, but 

 there is no cardinal process as in Rhynchotrema. 



In the section at Valcour Island, Camarotoechia first appears in 



