MARYLAND GEOLOGICAL SURVEY 587 



Occurrence. MATAWAN FORMATION. North shore Round Bay, Severn 

 River, Anne Arundel County. MOXMOUTH FORMATION. Millersville, 

 Anne Arundel County. 



Collections. Maryland Geological Survey, U. S. National Museum. 



C. Isodonta 



Superfamily PECT1NACEA 

 Family PECT1NIDAE 



Genus PECTEN Miiller 

 [Zool. Dan Prodr., 1766, p. 248] 



Type. Ostrea maxima Linne. 



Shell approximately equilateral, inequivalve, usually suborbicular, 

 auriculate; right valve, as a rule, the more convex, not adherent but 

 attached by a byssus; hinge line straight; resilium central, internal, tri- 

 angular ; interlocking grooves and ridges diverging from the apex of the 

 resilial pit; pallia! line simple; monomyarian; adductor impression 

 rounded, posterior. 



The earliest Pecten known is from the Cretaceous. The recent species 

 exceed two hundred in number and their distribution is world-wide. 



A. Shell not conspicuously inequivalve. 



1. External surface radially sculptured. 



a. Radial sculpture of more or less arcuate linear lirse. 



Pecten argillensis 



b. Radial sculpture coarse to fine but not linear nor arcuate. 



i. Adult shell exceeding 3 cm. in diameter; radials not sul- 

 cate, more or less scabrous, 30 to 40 in number. 



Pecten whitfieldi 



ii. Adult shell not exceeding 3 cm. in diameter; radials 

 medially sulcate, as a rule, but not scabrous, 12 to 18 

 in number Pecten venustus 



2. External surface not radially sculptured. 



a. External surface faintly sculptured concentrically. 



i. Adult shell exceeding 2 cm. in diameter. 



Pecten cliffwoodensis 

 ii. Adult shell not exceeding 2 cm. in diameter. .Pecten conradi 



b. External surface smooth, adult shell not exceeding 2 cm. in 



diameter Pecten simplicius 



B. Shell conspicuously inequivalve Pecten guinquecostatus 



Etymology: Pecten, a comb. A reference to the series of small tooth-like 

 spines placed on the margin of the shell at the byssal opening. 



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