584 GEOLOGY OF OHIO. 



distinct, elevation or ridge, formed by a thickening of the shell cor- 

 responding to that which forms the margin of the area. It serves no doubt 

 for the attachment of the adductor muscles, but is rarely well defined. 

 Commonly therefore the muscular area of the brachial valve is seen only 

 as a rounded area, fairly well defined, divided into halves by a strong 

 rounded median elevation or ridge, and that of the pedicle valve is as a 

 rule well defined only posteriorly. 



This species is found at the Soldiers' Home, Fauver, Huffman, 

 Beavertown and Centerville Quarries in the upper shaly courses; at 

 Todd'sFork in the ferruginous limestone; at Brown's Quarry in the lime- 

 stone; and at Hanover, Indiana. 



A smaller form, described as variety parva, Plate 25, Fig. 17a, b is 

 found in the limestones of the Soldiers' Home and Fauver Quarries, and in 

 the quarry in John Glaser's woods five and a third miles from Dayton, 

 on the Brandt pike. They probably do not even form a distinct variety, 

 but are only the young stages of this species; but why in the middle 

 limestones of the Soldiers' Home quarry these small specimens should 

 have been so common, and the fully developed specimens either absent 

 or at least exceedingly rare, could not be well explained. In the upper 

 shaly courses at the Soldiers' Home the full-grown forms just mentioned 

 are very common. 



Or this (Rhipidomel/a) hvbrida, Sowerby. 



(Plate 25, Figs 10 a, b) 



Of this species no interiors were found. At least no interiors were 

 found which resemble published figures of this species, which would 

 serve as a means of identification, had the existence of this species in the 

 Ohio Clinton otherwise escaped our attention. This may be accounted 

 for by the fact that even the exteriors of this species are quite rare, hav- 

 ing been found but rarely in the upper shah- courses at the Soldiers' 

 Home Quarry and in the limestone at Fauver's. The species is readily 

 recognized by the commonly almost equal convexity of the valves, and 

 the fact that in the more mature individuals the anterior half of the ped- 

 icle valve is more or less flattened. To a certain extent, moreover, the 

 fine radiating plications are finer and closer than those of Orthis e.eganhda. 



At Brown's Quarry is found a form also with fine radiating plica- 

 tions, and with the valves about equally convex. The anterior half of 

 the pedicle valve, however, does not show flattening. But since our 

 specimens are not more than 11 mm. long it is probable that this represents 

 either a less developed form or merely the young of Orthis hybrida, cor- 

 responding to the variety parva of Orthis eleganiula. 



At Brown's Quarry are also found forms in which the hinge area 

 does not seem to be sufficiently elongated to form postero-lateral angles 



