Report of the state Geologist. 555 



Pedicle-valve with a more or less elevated umbo, which may 

 vary in height in a given species; umbonal cavity divided into 

 two chambers by a longitudinal septum. Cardinal area large; 

 deltidial ridges and deltidial callosities not always distinctly 

 developed; cardinal slope usually well defined, merging anteriorly 

 into tin' cardinal buttress, or umbonal septum; in some species 

 this slope, or subtriangular area, is longitudinally divided by a 

 furrow. The crescent is sharply impressed over the cardinal 

 slope; terminal scars broad and distinct. Platform more or less 

 developed ; divided by the cardinal buttress, which extends 

 nearly or quite its entire length. Muscular impression on the 

 platform very large, covering the entire surface, but scarcely 

 resolvable into separate scars. Anterior septum faint, usually 

 imperceptible. Pallial sinuses sometimes clearly defined. 



Brachial valve with the umbo seldom conspicuously developed, 

 the umbonal cavity being usually simple and deep, but sometimes 

 filled by a deposition of shelly matter. Crescent sharply 

 impressed over the narrow cardinal slope, and produced into 

 broad, indistinct, composite, terminal scars. Platform usually 

 well developed and broadly excavate on its anterior walls, but 

 sometimes scarcely discernible. Anterior and lateral scars sharply 

 defined, giving a tripartite appearance to the platform. Anterior 

 septum more strongly developed than in the opposite valve. 



Type, Monomerella prisca, Billings. 



Distribution, Upper Silurian. Eight species are known, six 

 of which occur in the Niagara and Guelph faunas of North 

 America, two on the island of Gotland and one in Livonia. 



Rhinobolus, Hall. 1871. 



(Plate 2, Figs. 7, 8.) 



Synonym; Obolellina (jpartim), Billings. 1871. 



General form of the shell as in Trimerella. Pedicle-valve, in 



the typical species, with an elevated, solid beak, having a broad 



and high deltidium, divided into a relatively narrow central area 



(which may be concave or convex), and broad deltidial ridges. 



Crescent and terminal scars as in Trimerella. Umbo-lateral 



scars sharply defined. Platform very low and incompletely 



developed, consisting of a broadly Y-shaped elevation, with a 



raised anterior margin, and surface sloping abruptly backward 



107 



