472 MEMOIRS OF THE CARNEGIE MUSEUM. 



form (Subgenus Lariroslns Simpson), which we may with considerable confidence 

 assume does not belong here, until the contrary has been positively demonstrated. 



Thus restricted, the genus Diplodon from our point of view falls in Simpson's 

 arrangement into two subgenera, Diplodon (Simpson, 1914, p. 1226) and Cydomya 

 (Simpson, 1914, p. 1278), each subdivided into groups. The characters of the 

 shells of these groups run into each other very insensibly, and are found in various 

 combinations. The subgenus Cydomya is somewhat better defined, inasmuch as 

 the rounded outlines of the species assigned to it contrast rather markedly with 

 most of the elliptical, somewhat elongated shells, assigned by Simpson to typical 

 Diplodon. But there are intergrades even here. Typical Cydomya (of the fune- 

 bralis-type) is irregularly circular, with rounded angles (obscurely pentagonal), 

 and has a short and high shell, with the greatest height situated in the middle of the 

 shell, at about the middle of the ligament. Simpson included in it as species 

 Jontainianus, and gratus, which I regard only as higher and shorter forms belonging 

 to Diplodon. They have the greatest height of the shell not in the middle, but 

 more posteriorly, behind the ligament, and thus their outline is distinctly oblique 

 and subtrapezoidal, although approaching the rounded shape. 



Simpson, indeed, says in addition, that the beak-sculpture in typical Diplodon 

 consists of "unbroken ridges," and that in Cydomya it is "irregularly radial." 

 In both cases this holds good only for certain species, and cannot be used as a 

 generally distinguishing character. 



Subgenus Diplodon Simpson. 

 Simpson, 1900, p. 873; 1914, p. 1226. 



Shell more or less elongated; elliptical, ovate, or subtrapezoidal in outline; 

 when short, more or less angular and distinctly oblique, with the greatest height 

 behind the ligament, but not subcircular with the greatest height under the ligament. 



The arrangement into groups, as here given, does not rest upon that of Simpson. 

 It is largely made to suit my material, and does not claim to be final. We may ex- 

 press the essential differences in the following key, but with the distinct under- 

 standing that there are transitions between the groups, which are hard to i>lace. 



Key to Groups in Genus Diplodon. 



a. Shell .straight, not oblique, i.e., the longest axis is nearly parallel to the ligament. 



6. Beak-sculpture covering a considerable part of the disk; ridges, chiefly the posterior, rather 



heavy Group of D. hijlwus. 



bb. Beak-sculpture more or less develojied, rarely covering a considerable part of the disk; ridges 

 not very heavy and ratlier uniform. 



