28 Braclviopoda of the Cincinnati Group. 



equals the greatest breadth of the valves, but may be a little less or a 

 little greater than the breadth. Its lateral slopes are always more 

 abrujit and much less compressed than the laticosta, but like that 

 form its mesial sinus is large, and very profound, with angular margins, 

 and its mesial fold strongly elevated. In old individuals the valves 

 become often remarkably gibbous, the convexity exceeding the length 

 and nearly equaling the breadth [the old individuals here referred 

 to in Prof. Meek's description are usually found clustered together, 

 and I believe they are the same defined by Mr. James, as 0. crassa, 

 in vol. i., page 20, of this Journal, January, 1874] ; being much in- 

 /3reased by the prominence of the mesial fold, and the elevation of 

 the plications forming the margins of the sinus. It also differs in 

 having but a single, strong plication in the sinus, and only two on 

 the mesial fold. All of the plications are simple, and excepting the 

 occasional rudimentary ones, quite coarse, prominent and angular ; 

 the number on each lateral slope being constantly five or six. 



Found from 250 feet above low water-mark to the top of the hill^, 

 back of Cincinnati, and at other places. 



For Oiihis cypha (James), see vol. i., page 20, of this Journal, 

 January, 1874. 



Orthis acitfi/'irato— (CoNEAD, 1842). 



The most common form is much extended on the hinge line, which 

 usually terminates in acutely angular or even mucronate lateral 

 extremities, thus causing the breadth to be sometimes twice, or even 

 three times the length of the valves. Others have the hinge not more 

 than one fifth greater than the length. In all of its variations of propor- 

 tional length and breadth, it agrees in having three, or very rarely 

 four, simple angular plications in the bottom of the sinus, and four or 

 very rarely five on the mesial fold, which is always rounded, and 

 comparatively little elevated. All of its plications are simple, and 

 smaller, and more numerous than those of the lijnx, latlcosta, dentata (f), 

 there being on each side of the mesial fold and sinus from 11 to 18, 

 making the entire number from 26 to 40 on each valve. The speci- 

 mens with extended lateral extremities have the most j)]ications, but 

 only about the same number reach the beaks, as on those less dilated, 

 because they run out on the lateral extensions on the hinge line. 



This form becomes quite gibbous with age, the gibbosity being, 

 generally, most obvious in the specimens least extended on the hinge 

 line, some of those with the most produced lateral extremities having 

 the middle portions of the valves qiiite as convex as any of the others 



