82G pR(i<-i-:i-:inxas OF THE NATIONAL MUSEUM. VOL.XX. 



PECTEN M tiller, 1776. 



Pecten (1st section) KLEIN, 1753 + Vola. 



Pccten MULLER, Prod. Zool. Daii., 1776 (pars). DAC'OSTA, 1778. BOLTEX, 1798 

 (restricted). CUVIER, 1798. LAMARCK, Syst., 1801. VERRILL, Trans. Conn. 

 Acad., , pp. 56, 89, 91, 1897. 



.fatih-ii SCHUMACHER, 1817. BALL, 1886 (pars). FISCHER, 1887. 

 I'ola H. :ind A. ADAMS ("after KLEIN), 1858. STOLICZKA, Mem. Geolog. Survey <(' 



India, Cretaceous Pelecypod Fauna, III, p. 426, 1871. ZITTEL, 1X81. 

 To/a -f- Ja-nlra CIIENU, 1862. 



Type. I'ecten nuiximns (Linnaeus). 



Since Bolten, in 1798, definitely restricted the name Pecten to this 

 group, his restriction has precedence over that of Schumacher. 



The shells are generally large and heavy, and the valves are very 

 unequal, even when very young. The right valve is strongly convex 

 with a large and much incurved umbo and beak, while the left valve 

 is llat or even concave. It is usually smaller than the right, and shuts 

 closely inside of its scalloped margin, and its umbo is nearly or quite 

 obsolete. The auricles are of moderate size and not oblique, and in the 

 right valve they are strongly convex or excurved in the middle. This 

 valve has a sinuous, excurved byssal notch, with obsolete pectinidial 

 teeth. The surface of both valves has strong radial ribs interlocking 

 at the margin. Internally there are angular, thickened, and fluted 

 radial ribs opposite the external grooves; these ribs become more 

 prominent and bicarinate or fluted near the margins. 



AMUSIUM Bolten, 1798. 



Amusiitm BOLTEN, 1798. MUHLFELDT, 1811. SCHUIIACHER, 1817. WOODWARD, 

 I860. DALL, 1886. VERRILL, Trans. Conn. Acad., X, pp. 57, 90, 92, 1897. 



Amiissium H. and A. ADAMS, 1858 (pars). STOLICZKA, Mem. Geolog. Survey of 

 India, Cretaceous Pelecypod Fauna, III, p. 426, 1871. FISCHER, 1887. 

 ZITTEL, 1881. 



Pleiironectia SWAIN, 1840. CIIENU, 1862. 



Type. Amusium pleuronectes (Linnreus). 



In this very distinct genus the shell is round, thin, nearly smooth, 

 and strongly compressed. The surface is often polished, sometimes 

 lightly radially striated, never strongly ribbed. The margins are sim- 

 ple and thin. The valves may be a little unequal in convexity and 

 usually differ in color and somewhat in sculpture. The valves come 

 together ven trally, but usually gape at both ends. The auricles are 

 small, symmetrical, nearly equilateral, often with lateral crune; the 

 byssal notch is small or absent, pectinidial teeth nearly or quite abor- 

 tive. The adult probably has no byssus. Hinge-plate simple. Interior 

 of valves strengthened by a number of raised divergent ribs, or line, 

 independent of any external sculpture. 



