A. E. Verrill — SAidy of the family Pectinidce. 57 



Chenu and some other writers have made two generic divisions of 

 the one-sided Pectens, viz : Vola, type P. maximus L. ; and Janira, 

 type P. atavus. The latter is a fossil of cretaceous age. No import- 

 ant diagnostic characters have been pointed out, however. These 

 two names, as originally used, were absolutely synonymous. 



This genus occurs in rocks as early as the lower Cretaceous. The 

 extinct group Neithea is closely allied. It differs only in having 

 conspicuoiis transverse incisions on the upper cardinal ribs. Pecten 

 has a wide distribution in all tropical and subtropical seas. P. zic- 

 zac L. and P. heniicyclicus Ravenel occur in the West Indies ; P. 

 dentatus is found on the Pacific coast of America, from the Gulf of 

 California to South America. 



Amusium Bolten. Type, A. pleuronectes (Linne). 



Amusium Bolten, 1798 ; Muhlfeldt, 1811; Schumacher, 1817; Woodward, 1866; 

 Dall, 1886. 



Amussium {pars) H. and A. Adams, 1858; Stoliczka, op. cit., p. 426, 1871; 

 Fischer, 1887; Zittel, 1881. 



Pleuroneciia S^am., 1840; Chenu, 1862. 



In this very distinct generic group the shell is round, thin, nearly 

 smooth and strongly compi-essed. The surface is often polished, 

 sometimes lightly radially striated, never strongly ribbed. The mar- 

 gins are simple and thin. The valves may be a little unequal in con- 

 vexity and usually differ in color, and somewhat in sculpture. The 

 valves come together ventrally, but usually gape at both ends; The 

 auricles are small, symmetrical, nearly equilateral, often with lateral 

 crursB ; the byssal notch is small or absent, pectinidial teeth nearly 

 or quite abortive. The adult probably has no byssus. Hinge-plate 

 simple. Intei'ior of valves strengthened by a number of raised diver- 

 gent ribs, or lirae, independent of any external sculptui'e. Accord- 

 ing to Dr. Dall, the foot in A. pUxironectes = Mortoni is large, with 

 " a spade-shaped tip and well-developed sucker, with moderate 

 stem." In A. JDalli it is described by him as having the " sucker 

 large, hood-shaped, thin-walled and darkly pigmented, with a broad 

 base, abruptly enlarged from a very slender stem." The stem is 

 strongly grooved, the expanded cord is hollow and forms " an exag- 

 gerated and efficient sucker." The ocelli are without pigment. 



In the latter species there is but a single gill on each side, " fur- 

 nished with long separate filaments, much as in Diniyay^ 



' Owing to these important differences in the structure of the gills and fool, it 

 seems necessary to place this species in a distinct genus, for which I have proposed 

 the name Paramusium. (See p. 72.) 



