11 



est height. Moreover, in what appears to he a typical, though pos- 

 sibly an extreme form of M. compressus, with the test preserved, (an 

 outline of which is represented in wood-cut, fig. 1) the umbo is com- 

 pressed and comparatively narrow, the beak is curved very slightly 

 downwards, there is no lunule, and the anterior end projects beyond 

 the beaks as a broadly rounded lobe. Fig. le, on Plate Ixii of the 

 second volume of the Palseontology of New York, which is described 

 by Prof Hall as " a cast of a specimen " of M. Canadensis " somewhat 

 distorted by pressure which has projected the lower anterior end some- 

 what beyond the beaks above," represents perfectly a normal and 

 undistorted cast of this form of M. compressus. 



In the most typical form of the true M. Canadensis, when the shell is 

 preserved, the exceedingly broad and tumid umbones are anterior, 

 terminal, and overhang the abrupt downward and backward slope of 

 the lower part of the anterior end. The beaks, too, which in con- 

 sequence of the enormous breadth of the umbones, are placed two- 

 thirds of the way from the dorsal margin to the base, are recm-ved 

 and strongly hooked, and under them there is a rather deeply exca- 

 vated heart-shaped lunule whose width is greater than its height. 



Fig. 2. Anterior end of left valve of a specimen of a Megalomus which is inter- 

 mediate in character between M. compressus and M. Canadensis. 



But between these two extremes there occur almost every inter- 

 mediate gradation, both in the amount of convexity as compared with 

 the height and in the outline of the shell, especially at the anterior end. 



