CLASS 1 



PELECYPODA 



457 



thus Heparated usiially discrepaiit. Beaks inesogyrate, lateral teetli fluted, inuscular 

 scars biittressed by feel)le ridges. Abuiidaiit in the Trias. 



Siil)genus : Myoijhorio'psis Wüliriii. {Astartopsis Wölir.). Trias. 



Trigonia Brug. (Figs. 703-707). Surface sculptured witli iiodulose ribs or rows of 

 pustiiles, tlie 2>osterior dorsal area usiially discrepaiit witli tlie rest. Beaks opistho- 

 gyrous, nearly teriuinal ; teetli striated ; addiictor scars stroiig, with biittressiiig ridges. 

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



Section D. Isodonta Fischer. 



Superfamily 7. PECTINACEA Reeve. 



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

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

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

 filaments loith or withoiU a reßected limb ; mantle lobes free, without 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- 

 oecious : marine. 



Family 33. Pectinidae Lamarck. 



Shell inequivalve, inequilateral, auriculate, usually closed, 

 free; area amphidetic or obscure ; ligam.ent obsolete extern- 

 ally, the immersed portion forming an internal resilium, 

 provinculum taxodont in the very young, obsolete later, the 

 crural teeth feeble or not developed. Silurian to Recent. 



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

 radially sculptured. Hinge margin long, feebly auriculate; 

 ligament in numerous shallow grooves radiating to the 

 amphidetic margin of the area. Silurian to Carboniferous. 



Subgeiiera : Ficrineo2)ecten Hall 

 2)ecten Hall). Devonian. 



monomyarian, us 



ually 



Orhipecten Frech {Lyrio- 



Fio. 708. 



Ä viculopecten 

 Goal Measures 

 phalia. 



]Hipyraccus Sow. 

 Werden, West- 



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 Müller {Vola Mörch ; Janira Schum. ; Neithea 

 Drouet) (Fig. 709). Shell nearly equikteral, 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 ; Ijyssal notcli 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. Thoy have been divided into an ex- 

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

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

 most familiär of the groups thus named are as follows : — 



Chlamys Holten {Pallium Schum.; Decadopedcn Rüpp.) (Figs. 710, 711). Shell radially 



l'ecten quinquecostata Sow. Ceno- 

 manian ; Rouen. i/i- 



