42 
PALAEONTOLOGY OF NEW YORK. 
Cedarburg and Grafton, Wisconsin, show not only the high area and deep 
chambers of the Canadian types, but deviations therefrom, which, if found in 
specimens unassociated with the 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 broad, 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 ai T ea, equally broad and high, is gently 
incurved, as in Rhinobolus, and is transversely striated, but has only very faint 
evidence of longitudinal subdivision. In M. Greenii 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, but 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, we have an example of this umbonal 
thickening carried to an extreme. 
The platform, never. so 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. 
