MOUNTAIN LIMESTONE FOSSILS. 



203 



of the beaks ; ligament confined to a narrow simple facet on 

 the hinge-margin. 



These shells difier from some of the short-winged group of 

 Avicula (or Pteria), to which they are most allied, by the obli- 

 quity of the body of the shell being towards the anterior instead 

 of the posterior side — the reverse, in fact, of what occurs in nearly 

 all shells except the Limce, There are many species in the car- 

 boniferous limestone, to which formation the genus seems at pre- 

 sent confined, unless the Pterinea posidoniaformis (M'Coy), (Syn. 

 Sil. Foss. of Ireland, t. 2. f. 10) of the Upper Silurian strata 

 belongs to it. 



AvicuLOPECTEN (M'Coy), n. g. 



Gen. Char. Inequivalve, more or less inequilateral, straight, or 

 slightly extended ob- 

 liquely towards the pos- 

 terior side ; anterior ear 

 flattened, smaller than 

 the posterior, sharply 

 and deeply defined, with 

 a deep notch in the right 

 valve between it and the 

 body of the shell for the 

 passage of the byssus ; 

 posterior ear slightly 

 pointed, extending about 

 as far as the margin of 

 the shell, defined or not ; 

 ligament and cartilage Internal cast oi Aviculopecten. 



confined to a narrow facet along the hinge-margin, no medial 

 cartilage-pit ; muscular impression and pallial scar as in Pecten. 



It was only on seeing the fine suite of fossils from the dark 

 limestone of liowick, Northumberland, recently presented by the 

 Rev. Mr. Jenkins to Prof. Sedgwick, and now in the collection 

 of the University of Cambridge, that I recognized the characters 

 by which the great bulk of the so-called Pectens of the middle 

 and upper palaeozoic rocks are distinguished from the true Pectens 

 of the more recent formations and present sea. In the present 

 fossils the posterior ear is largest, thus differing in an external 

 character from Pecten and approaching Avicula, an affinity greatly 

 increased by the internal structure exposed by the Lowick (and 

 some Irish) specimens, showing that there is no mesial ligamen- 

 tary pit beneath the beak as in the former genus, but, as in the 

 latter, the ligament is confined to the hinge-margin, while in 

 general form and little or no obliquity of the shell the resem- 

 blance of the species to Pecten is so very striking that most 



