ANIMALS. 169 



Diagnosis. — " Shell subrhomboidal, arched, smooth ; front wing acute, hinder one 

 obtuse angulate."^ (Miinster.) 



Probably Count Miinster drew up the above diagnosis from a smooth variety, or a 

 specimen divested of its testaceous covering, — a rather common case ; as this species 

 is quite as much striated as the last. 



Dr. Geinitz has merged BaJcevellia antiqua into B. ceratophaga ; but there is clearly 

 sufficient difference between them to warrant their separation. The present species 

 differs from B. ceratophaga in being a more tumid shell ; in having the valves thicker, 

 the umbones more divaricated, the hinge-areas larger, the sinus in the wing-areas 

 much smaller, the wing less produced, the byssal or pedal sinus only slightly marked, 

 the incremental striae more regular, and the anterior lobes and wing-areas decidedly 

 less obviously separated from the convexity of the valves. It also attains a larger 

 size. My largest specimen is an inch and a quarter in width, that is, in the direction 

 of the hinge, and seven eighths of an inch in length. 



Owing to the thickness of the valves of Bakevellia antiqua, the muscular impressions 

 are so strongly marked, that casts occasionally display them in the most instructive 

 manner. The cast represented in PI. XIV, fig. 33, particularly enabled me to decide 

 that the species had no relation to the genus in which it had usually been placed : the 

 anterior and posterior adductor muscular impressions {a, b) and the palHal line (c) are 

 so well marked, that there can be no doubt of its being a true Dimyarian. 



Notwithstanding the thickness of the valves, both are strengthened internally on 

 the anterior side of the umbonal cavity with a slight ridge, which posteriorly bounds 

 the anterior adductor muscular impression. 



Bakevellia antiqua has some resemblance to young specimens of the Carboniferous 

 Gervillia lunulata, Phillips. It is in some localities a rather variable species, particu- 

 larly in the neighbourhood of Manchester, where specimens occur varying much in the 

 divarication of the umbones. Captain Brown has been led to regard these varieties, 

 from an examination of some imperfect and in many cases distorted casts, as different 

 species ; but I feel persuaded, after carefully examining the original specimens, 

 including others contained in various collections, that they are all referable to the 

 present species. 



Bakevellia antiqua is a widely distributed species, and appears to be characteristic 

 of every member of the Permian system. It occurs in the Shell-limestone of 

 Humbleton Quarry, Hylton North- Farm, South wick-lane House, Dalton-le-Dale, 

 Ryhope Field-House Farm, Tunstall Hill, Silksworth, and Castle Eden-Dene ; in the 

 probably contemporaneous Breccia at the north end of Black- Hall Rocks ; and in 

 the lowest beds of Whitley Quarry. It is found in the Permian Marls at Bedford, 

 Collyhurst, and Newtown near Manchester (Binney) ; also at Woodhall, Stubbs Hill, 



1 " Avicula testa subrhomboidali fornicata Isevi, ala antica acuta postica obtusangiila." (Goldfuss, 

 Petrefacta, 2d part, p. 126.) 



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