1907.] 



NATURAL SCIENCES OV IMIILADKLI'llIA. 



411 



fig. 9). Ill .1/. helium the valves are a little wider, nieasurinp; 0.75 x 0.3^ 

 mm. Ill hotii species a ritlge or angle close to the occludent bonier of 

 the scutum defines a narrow and nearly flat ventral area; and towards 

 the base a smooth sunken area separates the primitive valves and the 

 sculptured portions of the scuta, as shown in figs. \b, 'lb. 



The scuta, when removed, are seen to differ in various respects. 

 The occludent border is more bowed in .1/. bdlum, and the basal por- 

 tion, below the umbonal tooth, is wider than in .1/. minus. In ])oth 

 species the umbonal tooth is stout and higii in l)()tli valves. There is 

 no socket in the carinal margin for the reception of tiie tooth of the 

 carina, such as Hoek described and figured in M . striatum (see fig. 3). 



h m 



Fig. 4. — M. minus, carina in i'entral Fig. .5. — M. helium, carina in ventral 

 and lateral views. and lateral views. 



The carina in M. minus (fig. 4) has much narrower sides, which are 

 more expanded at the baso-lateral angles. The basal margin has a 

 rounded median projection, and the articulating teeth are larger. In 

 M. bdlum (fig. 5) the basal margin is simply arcuate. In other respects 

 the carinas are similar. The shape of the base of the carina affects the 

 shape of the peduncular foramen, as view-ed from the ventral side (see 

 figs, lb and 26). The difference of shape is equally pronounced if M. 

 minus is compared with young specimens of M. helium, of equal size. In 

 M. minus the basal angles of the carina are more produced, and spread 

 apart farther than in M .bdlum. The septum across the cavity is situated. 



