42 PALAEONTOLOGY OF NFU' YORK. 



Cedarburg and Grafton, Wisconsin, show not only the liigh area and deep 

 chambers of the Canadian types, but deviations therefrom, which, if found in 

 specimens unassociated with tlie intermediate forms, might be regarded as of 

 distinct specific value. We have, on a preceding page, called attention to 

 essentially similar variations in the species Dinobolus Conradi. 



The cardinal area of the pedicle-valve in M. prisca is always erect, and its 

 subdivisions are sharply defined; a l)road, usually convex though depressed 

 deltidium is bounded by elevated deltidial ridges merging into the areal 

 borders, and has the deltidial callosities or articulating (?) apophyses more 

 strongly developed than elsewhere noticed among the Trimerellids. On the 

 other hand, in M. Ortoni, the cardinal area, equally broad and high, is gently 

 incurved, as in Ruinobolus, and is transversely striated, but has only very faint 

 evidence of longitudinal subdivision In M. Grecnii and M. ovata, this area 

 is low, smooth, and not subdivided, but is continued about the margins into a 

 broad surface of contact with the other valve. In the brachial valve of M. 

 prisca and M. Kingi the cardinal area is slightly if at all developed, the beak 

 being marginal; while, in M. Greenii, it is somewhat elevated, and, in M. Egani, 

 very prominently developed. 



The development of the umbonal chambers depends essentially on the height 

 of the umbo, and the degree of its thickening; in M. prisca always extending 

 above the hinge-line, in M. ovata and M. Kingi they reach to the hinge, and, in 

 M. Greenii, rarely appear to extend so far. The umbonal cavity of the brachial 

 valve is subject to even greater variation, but this is more largely an individual 

 than a specific divergence. In the thin-shelled species, M. Greenii, this cavity 

 is invariably deep, l)ut in M. prisca it appears to be frequently thickened by 

 internal deposition. This feature is also seen in M. Kingi, and in the single 

 valve representing the species, M. Egani, Ave have an example of this umbonal 

 thickening carried to an extreme. 



The platform, never m strongly developed as in Trimerella, nor furnished 

 with vaults, is usually a conspicuous feature, but in M. Greenii it is almost 

 obsolete, its position being evident only from the slightly thickened muscular 

 scars. 



