410 



PROCEEDINGS OF THE ACADEMY OF 



[Sept. 



carina. M. minus is slightly narrower than M. helium throughout. The 

 dimensions of the type were not given by Dr. Annandale in his original 

 account, but the name implies that it is smaller than M. striaium, which 

 has a capitulum 11 mm. long. The figure of his type, subsequent!}^ 

 published, is said to be enlarged six diameters, which would make 

 the capituhun about 9 mm. long. The largest specimen I have 

 seen, a 9 bearing a large mass of eggs, measures 6.5 mm. in length of 

 capitulum. M. helium attains a much greater size. The type is 14.5 

 mm. long, and I have seen a great number of specimens from many 

 localities with capitula from 12 to 15 mm. long. 



Fig. 3. — a, internul view of the left scutum and tergum of M. minus, X 10, and 



6, M. helium, X 5. 



The lateral borders of the peduncular foramen, in a lateral view of 

 the capitulum, are nearly straight in M. helium (fig. 2, po), but in M. 

 minus these margins arch outwards more or less strongly (fig. 1, po). 



The primitive valves, visible at the umbones, are convex, densely 

 cellular in both species, irregularly trapezoidal in outline, with sub- 

 parallel occludent and dorsal borders, the latter much the shorter. The 

 anterior (basal) end is squarely truncate, the posterior end oblique. In 

 M. minus the primitive valves measvtre 0.66 mm. long, 0.25 wide (pl.31 , 



