78 A, E. Verrill — Study of the family JPectinidce. 



each rib, while the outer ones are small and much crowded. The 

 ocelli are numerous, unequal in size, the larger ones at first alternate 

 evidently with the primary tentacles and are opposite the external 

 sulci. The ocelli are very brilliant in life. 



This species is abundant in shallow water, especially of bays and 

 sounds, from Cape Cod to Florida. It is used extensively as food. 

 The adductor muscle is the only part utilized for this purpose. 



Chlamys (Placopecten) Clintonius (Say) Ver. See p. 69. 



Peden Magellanicus f Gmelia, Syst. Nat., p. 3317, 1788, (a bad and misleading name 

 if applied to this species). Lamarck, Anim. sans vert., ed. 2, vii, p. 134. Gould, 

 Invert. Mass., ed. i, p. 132. Ball, Bull. Mus. Comp. Zool, p. 216, 1886. 



Peden Clintonius Say, Journ. Acad. Nat. Sci. Philad., iv, p. 124, pi. 9, fig. 2, 1824. 

 Verrill, Trans. Conn. Acad. Sci., vi, I, p. 261, 1884. Rep. U. S. Com. Fish and Fish- 

 eries, for 1883, p. 577. 



Peden tenuicostatus Mighels and Adams, Proc. Boston Soc. Nat. Hist., i, p. 49, 1841 ; 

 Boston Journ. Nat. Hist., vol. iv, p. 41, pi. 4, fig., 7, 1842 (the young of the smooth 

 variety). Gould, op. cit , ed. 2, p. 196, fig. 494. Verrill, Invert, of Vineyard Sound, 

 etc., p. [696] 402, 1893. 



Peden princeps Emmons, Rep. N. C. Geol. Survey for 1858, p. 280, fig. 198 (fossil 

 form). 



Amussium Magellanicum H. and A. Adams, Genera Moll., ii, p. 555, 1858. 



Peden {Pseudamusium) Miilleri Dall, Bull. U. States Nat. Mus., No. 37, p. 34, 1889 

 (the young). 



Plate XVII. figs. 1-7. Plate XX. figs. 7-8a. Plate XXI. figs. 1-1 a, 2, 2a. 



When very young this species is nearly smooth on both sides, but 

 when about 3-4™™ in length, it develops small, regular, raised ribs 

 over the whole surface of the upper valve, and usually at both ends 

 of the lower one, with intervening camjDtonectes sculpture. (PI. 

 xvii.) 



These small ribs increase in number, but not much in size, until 

 the shell is 2 inches or more in diameter. After that size, in the 

 greater number of shells of the northern variety, they decrease in 

 size till the upper valve becomes nearly smooth, or has only linear 

 riblets. But in some northern examples and in many of those taken 

 in deep water south of Long Island, the small ribs continue regularly 

 over the whole surface of the upper valve, and are more or less 

 roughened by the raised edges of small concentric lamellaa or lines 

 of growth, sometimes becoming more or less finely cancellated. 

 There are no corresponding internal ribs, except in extreme exam- 

 ples of this variety, near the margin, and the edge of the shell is 

 usually only slightly crenulated by the riblets, while the edge of the 

 lower valve is essentially plain and sharp. (PI. xvii. figs. 5-7.) 



