554 Forty-fifth Report on the State Museum. 



especially over the crown, where it makes a sharp posterior angle 

 beneath the beak, sloping thence very gently, being almost trans- 

 verse for about the widen of the platform, then bending quite 

 abruptly and being slightly interrupted at the position of the 

 terminal scars, which are more approximate than in the opposite 

 valve. In the type-species, the crown of the crescent, as usually 

 developed, is a sharply incised, narrow furrow, bounded within 

 by a broad elevation sloping to the position of the subcardinal 

 scar, but not infrequently it is a ridge, bounded in front and 

 behind by a deep furrow. This difference in appearance is due to 

 the varying development of the anterior furrow, which is some- 

 times quite suppressed. Platform narrower and more sharply 

 Y-shaped than in the opposite valve ; vaults usually very short 

 and abruptly conical. Lateral and central scars on the platform 

 conspicuous, the latter lying in a deep median furrow, at the front 

 end of which are sometimes seen the faint anterior scars. In the 

 line of this furrow, directly in front of the apex of the crescent, lies 

 the deep impression of the subcardinal muscle, which make a promi- 

 nent feature on the cast, suggestive of the filling of the umbonal 

 cavity, which it may be to some extent, although the apex of the 

 beak is actually quite remote from it. From this point to the cen- 

 ter of the crescent extends a short furrow, on either side of which 

 lies the impression of an accessory scar, probably correlative 

 with the umbo-lateral scars of Ehinobolds and Trimerella. 

 Pallial sinuses more or less distinct. Median septum low, but 

 stronger than in the opposite valve. External surface smooth, 

 or with concentric, sublamellose growth-lines. 



Type, Dinobolus Conradi, Hall. 



Distribution. Lower and Upper Silurian. Nine species 

 have been recorded, one from the Lower Silurian of Brittany, 

 another from Esthonia, three from the same division in North 

 America, one in the Upper Silurian of North America, Gotland 

 and Bohemia, and three from the same division in Great Britain. 



Monomerella, Billings. 1871. 

 (Plate 2, Figs. 5, 6.) 



Shell usually thick; outline varying from elongate-ovate bo 

 subcircular. Surfaces of contact of the valves sometimes con- 

 spicuously broad. 



106 



