CHONETES. 185 



short and not very numerous, on the surface of the valve, but a row rises and follows 

 close to the cardinal edge. In the interior of the ventral valve there exists a tooth on 

 each side of the fissure ; the adductor muscular impressions are small, but prominent, and 

 separated by a median ridge or septum, which extends to about one third of the length of 

 the valve. Immediately under and outside of the adductor scars are two concave, sub- 

 quadrate, longitudinally grooved impressions, attributable to the cardinal or divaricator 

 muscles. In the interior of the dorsal valve, under the cardinal process, exists a mesial 

 ridge or plate, which extends to nearly two thirds of the length of the valve, and on either 

 side may be seen two well-defined muscular scars, which are produced by the quadruple 

 attachment of the adductor or occlusor muscle, while outside and in front of these are 

 situated the reniform impressions. The remaining surface not occupied by muscular im- 

 pressions is in both valves covered with small asperities. Dimensions very variable ; three 

 British examples have measured — 



Length 8, width II, greatest depth between valves 1£ lines 



6 9 1 



7 13 2 



Obs. This species is, in general, easily distinguished from its congeners by its strong, 

 simple ribs ; but although these are simple in the greater number of specimens, I have noticed 

 a tendency in a few rare instances to bifurcation and even interstriation close to the margin, 

 and especially so in one example from the gray Carboniferous limestone near Settle (fig. 7), 

 and which I had at one time distinguished by the varietal designation of interstriata. The 

 ribs are exceedingly variable in their number and strength in different specimens, and 

 every number between twelve and thirty can be counted in different examples, so 

 that the gradation is complete. It is highly probable, if not certain, that Prof. M'Coy's 

 (J. crassistria is nothing more than a variety of C. Buchiana with small ribs, and, indeed, 

 closely agrees with some Belgian examples of this last I received from Prof. L. de 

 Koninck. 



In England the species under description occurs in the Carboniferous limestone of 

 Settle, and again in shales at Malham Moor, in Yorkshire, the specimens in the limestone 

 possessing, in general, a greater number and smaller ribs than in the variety which is met 

 with in the shales. It has been found also at llutcheugh, in Northumberland. In 

 Scotland it occurs at Gare, in Lanarkshire, at 239 fathoms below " Ell Coal," and in 

 black Carboniferous shales {palp) at Bundoran, in Ireland. On the Continent it was 

 first discovered by Prof, de Koninck in the lower Carboniferous limestone of Vise, in 

 Belgium, where it is very rare. 



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