630 GEOLOGY OF OHIO. 



the beaks in casts of the interior. Other species of this type are the B 

 obesa and probably B. grandis, and one or two other species of this 

 report. Of these the first is larger and has coarser radii, while the 

 second is very much larger and has anteriorly flattened and widely separ- 

 ated beaks. B. intermedia is more gibbous, especially in the umbonal 

 region, and B. tenuistriata, Ulrich, from the upper beds of the Cincinnati 

 group, has much finer radiating striae, the number being not less than 

 seventy. 



Formation and localities: Utica horizon of the Cincinnati group, 

 occurring at localities in the vicinity of Cincinnati, Ohio, from 50 to 200 

 feet above the bed of the Ohio river. 



Byssonychia obesa, n. sp. 



Plate 45. Figs. 10-12. 



Shell usually of less than medium size, obese, ovate in outline 

 except where the full and prominent beaks project beyond the regular 

 curve ; hinge short, rounded behind ; byssal opening small, situated high, 

 the inner margin thickened so that a decided depression is formed in the 

 anterior side of casts of the interior. Radii from forty-two to forty-five; 

 length from 20 mm. to 33 mm.; height (from beak to base) from 26.5 mm. 

 to 40 mm.; thickness from 15 mm. to 25 mm. In one specimen that dif- 

 fers a little from the rest these measurements are respectively 27 mm. 

 38 mm. and 20 mm. 



This species rarely occurs except in the condition of casts of the 

 interior, but these are easily' distinguished from B. radiata Hall, sp., with 

 which collectors have generally identified them, by their more ventricose 

 valves, more rounded form, and deeper byssal excavation. From B. vera 

 the species is separated by its greater size, and more ventricose valves. 

 B. grandis is much larger and has carinated beaks, they being flattened 

 on the anterior side ; B. suberecta is a more erect shell and has a longer 

 hinge. Probably nearer than any of these species, at any rate in the 

 general expression of casts of the interior, is the Galena limestone spe- 

 cies described by Meek and Worthen as intermedia (Ambonychia intermedia). 

 Young specimens may be difficult to distinguish from that species, but I 

 have not yet seen any of B. obesa that were as small as the largest of B. inter- 

 media. Aside from the point of size, comparison shows that the Galena 

 species is, relatively speaking, higher, and that the outline is less rounded 

 especially in the postero-cardinal region. 



Two large specimens from a lower horizon ( about fifty feet below 

 the tops of the hills at Cincinnati,) may belong to an early variety of this 

 species. As however they had at least fifty radii we might be equally 

 justified in regarding them as examples of a gigantic variety of B. vera. 

 The length ot the larger of the two is about 60 mm. 



Formation and locality : Near the top of the Cincinnati group at 

 Richmond, Indiana, where it occurs in association with B. richmondensis. 

 Ulrich. and Ortonella hainesi, Miller, sp. 



