CARBDNIFEROTJS AGE. PERMIAN PERIOD. 49 



2. ATiculo-pectininge. 



Shell with posterior ear generally larger than the other ; hinge without a central cartilage pit ; cartilage 



apparently occupying a series of linear furrows in a more or less broad cardinal area. 

 Includes Aviculopecten, Streblopteria, and probably several undefined Palaeozoic genera. 



The Aviculopecten group seems to form a kind of transition from the Pectinidce 

 to the Pteriidce, and may possibly be distinct from them both, though it is evidently 

 more closely allied to the former than the latter. It seems to bear much the same 

 relations to the typical forms of the Pectinidce that the Pteriniince do to the typical 

 Pteriidce. 



Subfamily AVICULOPEOTmLN"^. 

 Genus AVICULOPECTEN, McCoy. 



Synon. — Avicula, Pecten, and Meleagrina (sp.), of various authors. 



Aviculopecten, McCoT, Ann. Mag. Nat. Hist. VII, 1851, 171 ; Brit. Pal. Foss. 1852, 392. 

 Etym. — Avicula and Pecten. 

 Examp. — Aviculopecten docens, MoCoT.' 



Animal unknown. Shell inequivalve, more or less inequilateral; straight, or 

 slightly extended obliquely towards the posterior side ; anterior ear flattened, 

 smaller than the posterior, sharply and deeply defined, with a notch in the right 

 valve between it and the body of the shell for the passage of the byssus;^ 

 posterior ear pointed, extending about as far as the margin of the shell, de- 

 fined or not; ligament confined to a narrow^ facet along the hinge margin; no 

 medial cartilage pit ; muscular impression and pallial scar as in Pecten. (McCoy.) 



We entirely concur with Prof. McCoy in separating this group of shells both 

 from Pteria (= Avicida) and Pecten. From the typical species of the latter of 

 these genera, they difi'er materially in having the cartilage extended along the 

 hinge instead of occupying a mesial pit under the beaks ; they also present the 

 external diff'erence of having the posterior ear larger than the other. From the 

 Pterias they are clearly separated by their more equilateral and less obhque form, 

 edentulous hinge, and the arrangement of the cartilage, as well as by their shell 

 structure. 



Some difference of opinion exists in regard to the family relations of this genus, 

 several authors placing it in the Aviculidce, and others with the Pectinidce. We 



• In first proposing this genus in the Annals and Mag. Nat. Hist. (VII, 1851, p. ITl), Prof. 

 McCoy does not say what species he regards as the type, though he figures, as an illustration of the 

 genus, a species (without a name), which seems to be his A. docens; at any rate it is clearly con- 

 generic with that form. 



^ Judging from Prof. McCoy's figures of Palaeozoic Pectinidse, in his Synopsis of the Carboniferous 

 Fossils of Ireland, the byssal sinus would seem to be sometimes as strongly defined under the anterior 

 ear of the right, as well as the left valve, in Aviculopecten ; or there is another genus presenting that 

 character. 



^ Some of our American species have a broad cardinal area, marked with distinct cartilage fur- 

 rows ranging parallel to the hinge line, or sometimes divaricately deflected under the beaks. 



7 June, 1864. 



