204 MOUNTAIN LIMESTONE FOSSILS. 



writers agree in placing them in that genus. The discovery of 

 this character fixes the zoological place of numerous carboniferous 

 shells constantly varying hitherto in the systems between Pecten, 

 Avicula and Meleagrina. 



Aviculopecten plano-radiatus (M^Coy). 



Sp. Char. Ovate, apical angle 80° in young specimens, 95° in 

 adults from an upward curve of the anterior side, length and 

 width nearly equal, gently convex ; beaks small, prominent ; 

 ears very deeply defined from the body of the shell by a narrow 

 very steeply inclined plane, left anterior one rotundato-qua- 

 drate, obscurely radiated, posterior ones longer, falcately 

 pointed, radiated by a few slender ridges crossed by the lines 

 of growth ; surface radiated with numerous ribs (thirty to 

 forty at 1^ inch from beak) which are smooth, broad, flat, more 

 or less irregular in width, and separated by a very narrow im- 

 pressed line towards the margin and body of the shell, but 

 nearer the beak they are sharp, narrow and alternately larger 

 and smaller ; the ears are sharply striated, parallel with the 

 margin, and have a few narrow distinct radiating ridges. 

 Width from 1|^ to nearly 4 inches. 



The radiations vary from 1^ line to |^ a line wide in different 

 specimens at the margin. This species difiers from the P. pla- 

 nicostatus (M'Coy) in its being oblique and the much greater 

 number of its ribs. 



Common in the carboniferous limestone of Derbyshire. 



{Col. University of Cambridge.) 



Aviculopecten Ruthveni (M'Coy). 



Sp. Char. Suborbicular ; apical angle about 110° in the adult, 

 from the upward curve of the sides, only 85° in the young ; 

 length slightly exceeding the width, tumid ; surface radiated 

 by about fifteen thick, rugged ridges, between each pair of 

 which are usually three smaller ridges, each pair separated by 

 a concave space about equal to the thickness of the ridges ; 

 ears large, the posterior one broad, extending as far as the 

 margin of the shell, with three or four distant radiating ridges 

 crossed by coarse lines parallel with the concave extremity; 

 anterior ear similar, but slightly smaller, both defined. Width 

 from beak to opposite margin about 2 inches, length (at right 

 angles to the width) about the same. 



Fragments of this species bear some resemblance to portions 

 of the Pecten ? quinquelineatus (M'Coy), but it is distinguished 

 by the much less number of the ridges, &c. I have dedicated it 



