CLASS I 



PELECYPODA 



457 



thus sejiarated usually discrepant. Beaks mesogyrate, lateral te(jtli fluted, muscular 

 scars buttressed by feel>le ridges. Abundant in the Trias. 



Subgenus : Myo'plwriofsis Wiihrm. {Astartopsis Wohr.). Trias. 



Tric/onia Brug. (Figs. 703-707). Surface sculptured with nodulose riljs or rows of 

 l)ustules, the posterior dorsal area usually discrepant with the rest. Beaks opistho- 

 gyrous, nearly terminal ; teeth striated ; adductor scars strong, with buttressing ridges. 

 Lias to Recent ; abundant in Jura and Cretaceous, very sparse in later horizons. 



Section D. Isodonta Fischer. 



Superfamily 7. PECTINACEA Reeve. 



Shell usually inequivalve, flabelKform, more or less auriculate, and monomyarian ; 

 shell structure sub-nacreous, corrugated, and rarely prismatic, occasionally tabular ; area, 

 lohen present, amphidetic ; ligament amphidetic, alivincular ; gills filibranchiate, free, the 

 filaments with or without a reflected limb ; mantle lobes free, loithout siphons, usually with 

 ■ ocelli, papillae, or other tactile prominences along the margin, and with an inner pro- 

 jecting lamina {curtain) near the margin, at right angles to the plane of the valves ; pallial 

 line simple ; foot small, usually sub-cylindrical, grooved, and byssiferous ; usually mon- 



FlG. 708. 



Avieulopecten "papyxaceus Sow. 

 Coal Measure.s ; Werden, West- 

 plialia. 



Family 33. Pectinidae Lamarck. 



Shell inequivalve, inequilateral, auriculate, usually closed, monomyarian, usually 

 free; area amphidetic or obscure; ligament obsolete extern- 

 ally, the immersed portion forming an internal resilium, 

 provinculum taxodont in the very yo^tng, obsolete later, the 

 crural teeth feeble or not developed. Silurian to Recent. 



Aviculopecten M'Coy (Fig. 708). Shell pectiniform, 

 radially sculptured. Hinge margin long, feeljly auriculate; 

 ligament in numerous shallow grooves radiating to the 

 amphidetic margin of the area. Silurian to Carboniferous. 



Subgenera : Pterineopecte^i Hall ; Orhipecten Freeh {Lyrio- 

 2)ccten Hall). Devonian. 



Grenipecten Hall (Pernopecten Winch.). Like Aviculo- 

 pecten, but with a taxodont hinge. Carboniferous. 



The preceding genera lead up -to the prototypes of Pteriidae 

 as a radical for the present family. 



Pecten Miiller (Vola Morch ; Janira Schum. ; Neithea 

 Drouet) (Fig. 709). Shell nearly equilateral, very inequi- 

 valve, sub -symmetrical, with well -developed, sub -equal 

 ears ; one valve (usually the right) more convex than the 

 other ; interior of the valves not lirate ; hinge with a 

 strong medial internal resilium, on each side of which 

 interlocking crural ridges and grooves radiate in the 

 adult ; byssal notch inconspicuous. Cretaceous to Recent, 



The above diagnosis is of the subgenus Pecten s. s. In a 

 wide sense all the species of Pecten are free and auriculate, and 

 without internal lirae. They have been divided into an ex- 

 cessive number of sections according to the superficial shell characters, but these rarely 

 march with anatomical differences, and cannot properly be regarded as of generic value. The 

 most familiar of the groups thus named are as follows : — ■ 



Ciilamys Bolten {Pallium Schum.; Decadopecten Riipp.) (Figs. 710, 711). Shell radially 



. Fro. 709. 



Pecten quinquecodata Sow. Ceuo- 

 manian ; Rouen, i/l- 



