CHAPTER XXIX: THE PEN SHELLS. SEA 

 WINGS. FIN SHELLS 



Family Pinnid/E 



Genus PINNA, Linn. 



Shell equivalve, wedge-shaped, oblique, thin, fragile, and 

 generally scaled; hinge lateral, toothless; umbones terminal 

 anterior; ligament in long groove; mantle double-fringed; foot 

 long, spinning strong cord. Osphradium developed into a fmger- 

 like process that can be extended beyond the shell margin. Large 

 genus in warm seas of Old and New World. 



The Rude Pen Shell {P. rudis, Linn.) is irregular in growth, 

 blistery, whitewashed on the orange-red surface. Semi-tubular 

 scales, large and few, adorn the indistinct radiating ridges toward 

 the margin. Length, 8 to lo inches. 



Habitat. — West Indies. 



The Noble Pen Shell (P. nobilis, Linn.) shows a startling 

 crowded array of erect, recurving prickly scales pointing in 

 various directions, on the outer portion of its triangular shell. 

 The tapering stem is scantily set with scales. The ground colour 

 is olivaceous at the umbo becoming rosy as it spreads outward. 

 The scales are yellowish. Length, 6 to 7 inches. 



Habitat. — Mediterranean. 



The Half-naked Pen Shell (P. seminuda, Lam.) is covered 

 with erect, delicate scales, that follow the lines of growth. On 

 the outer posterior area they are largest and are set on radiating 

 lines. Forward and approaching the umbones the scales become 

 gradually so minute and worn down that the apical half is bare. 

 This shell is thin, of smoky, transparent, horny texture lined 

 with pearl. The lips meet in a straight line. Length, 6 to 10 

 inches. 



Habitat.— North Carolina southward. 



The Prickly Pen Shell (P. muricaia, Linn.) is triangular, 

 with straight lips and incurving side lines. The obscure ribs are 



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