4<D NEW YORK STATE MUSEUM 



strongly raised in the middle. Umbonal cavity simple, deep and broad. 

 Platform very large, moderately elevated, extending three quarters the 

 length of the valve, with V-shaped anterior margin, and rounded, projecting 

 antemedian point ; slightly excavate on its anterior walls, often to the 

 extent of forming shallow platform vaults. Anterior septum a low, broad 

 ridge, more prominent than in the pedicle valve. Crescent appearing as a 

 well defined impression on the cardinal slope, its sides and ends curving for- 

 ward and being broader and less distinct. Transverse scars distinct, sub- 

 circular depressions. Umbo-lateral scars rather faint and small, lying at the 

 sides of the umbonal cavities. Both median and anterior scars of the plat- 

 form sharply defined, depressed below the lateral scars, which are obliquely 

 striate. 



Horizon. Lower Shelby dolomite, Oak Orchard creek. 



This large and ponderous shell has been found only at the locality 

 cited, but is there in very great abundance ; it is surpassed in number of 

 individuals only by Trematonotus alpheus and Poterioceras 

 sauridens. Some of the specimens are impressions of the exterior of 

 the shell. 



This species is in many respects closely related to M. prisca Billings, 

 whose characters have been described by several authors, 1 though nothing 

 is yet known of its exterior. So obvious is this relationship that M. nove- 

 boracum is evidently a local, more prosperous development of that 

 widespread Guelph species. It is, however, sufficiently different to neces- 

 sitate distinction. It is larger, the specimens attaining a length of 80 mm 

 and a width of 65 mm ; the umbonal cavities are, in most specimens, 

 absolutely and relatively much longer, some attaining a length of 35 mm ; 

 the umbo of the pedicle valve however is less acutely, but more broadly 

 tapering ; the cardinal area, pedicle surface and areal borders are therefore 

 relatively broader. The platform of the brachial valve extends farther 

 anteriorly, and the anterior septa of both valves extend as broad distinct 



1 See Billings, Paleozoic Fossils. Nicholson, Pal. Ontario. Whiteaves. Davidson. 

 Hall & Clarke. Pal. N. Y. v. 8, pt 1. 



