17(3 PALEONTOLOGY OF NEW YORK. 



musciihir scar, which is continued into a broader terminal impression. Plat- 

 form sharply elevated at its anterior edge, sloping rather abruptly backward. 

 Its surface bears the usual tripartite arrangement of the muscular scars. 

 Anterior longitudinal septum distinct. From directly behind the crescent 

 to the central muscular impressions, is a very broad, smooth, lunate slope, 

 occupying the position of the simple umbonal cavity usual in this valve of 

 MoNOMERELLA, and presenting the appearance of an abnormal deposition or 

 callosity. 



Though represented by a single specimen only, this form shows features not 

 elsewhere observed in the genus, viz., the great development of the cardinal 

 area, the composite character of the muscular impressions terminating the 

 crescent, and the broad posterior slope. 



From the Niagara group, near Grafton, Wisconsin. 



RHINOBOLUS Davidsoni, sp. nov. 



PLATE IV 15, FIGS. 10-12. 



Shell with a circular outline, except for the prominence of the beak. Pedicle- 

 valve with a moderately high, acuminate cardinal area, upon which the 

 central area, the deltidial ridges and areal borders have each about the same 

 width. Cardinal slope short ; crescent sharply defined ; terminal scars dis- 

 tinct. Platform broadly V-^haped, sloping less abruptly backward than in 

 R. Gallensis. Pallial sinuses faint. Brachial valve with marginal beak and 

 inconspicuous area. Crescent more prominently developed than in the 

 opposite valve, transverse over the crown as in Dinobolus; terminal scars 

 large. Platform more sharply angulated than in the pedicle-valve and some- 

 what more elevated, bearing conspicuous lateral and anterior scars. Longi- 

 tuilinal septum not pronounced. Pallial sinuses quite distinct. 



This species is based upon internal casts of opposite valves, which are in 

 entire harmony with each other and are readily separated from Dinobolus Con- 

 radi and the various species of Monomekella associated wdtli them at the same 

 locality. It is distinguished from the forms referred to Rhinoholus Galtensis, 

 Billings, by its more circular outline, less elevated pedicle-umbo, inconspicuous 

 brachial umbo, and broader, though less sharply elevated platform. 



Niagara group. Near Grafton, Wisconsin. 



