JURASSIC AND TRIASSIC BRACHIOPODA. 199 



186. Var. Dumbletonensis, Dav. Sup., PI. XXIX, fig. 5. 



Some years ago Dr. Wright sent me two examples of a largish Bhynchonella which 

 he had found in the Marlstone (Middle Lias) of Dumbleton, near Cheltenham. 



In external shape it is sub-pentagonal and nearly as broad as long ; the valves are 

 moderately convex and flattened. The dorsal valve, when viewed in profile, presents a 

 slightly convex curve at the umbo, and from there to near the front is nearly straight ; 

 close to the front the valve is bent downwards so as to meet the serrated margin of the 

 ventral valve. This valve is almost equally trilobed, the central lobe forming a broad 

 flattened mesial fold, which does not rise very much until near the front. In the ventral 

 valve the sinus is broad, flattened, and of small depth. The surface of each valve is 

 covered with about fifteen ribs, of which six or seven occupy the fold and sinus. The 

 beak is slightly incurved, foramen visible under its angular extremity. 

 Length 15, breadth 15, depth 10 lines. 



This form has all the appearance of being a flattened variety of Bh. tetraedra. 



187. Var. Northamptonensis, Walker. Sup., PI. XXIX, figs. 7 to 12. 



Mr. J. P. Walker knocked out of a single lump of stone some two hundred examples 

 of this small variety, ranging from 2 lines to 9 or 10 in length. He observed that 

 at 2 lines in length the shell was nearly circular and more or less globular or depressed, 

 the ribs being nearly all the same size, but with age the fold became more and more 

 elevated with two, three, four, or five ribs upon it, a corresponding number being like- 

 wise present in the sinus. 



It occurs in the Middle Lias of Bugbrook, near Weeden, in Northamptonshire. 

 Apparently adult examples are almost circular or rather longer than wide, spherical from 

 the dorsal valve being exceedingly convex and gibbous, while the ventral one is of much 

 less depth. In this and other respects it differs very materially from the var. Dumbletonensis 

 and B. tetraedra proper. In profile the dorsal valve presents a very convex and 

 regular curve, and the mesial fold is well defined and prominent, especially from about the 

 middle of the valve to the front. The beak is much incurved, so that the foramen can 

 be scarcely observed. The surface of each valve is covered with from fifteen to twenty 

 ribs, of which three to five occupy the fold and sinus. 

 Length 9, width 8, depth 7 lines. 



This form is intermediate between Bh. tetraedra and Bh. amaltheu 



