M0.n::n. /)/;/:/- JFJ TV?/,' UOLLUSCA VEHHI1.L J.V/> Hl'SIf. 831 



Although this group agrees with Ainu. viinn, in having internal ribs 

 without corresponding external grooves, it seems to be allied rather to 

 Cltltnnt/N. It may be regarded as a division of the latter in which the 

 external radial ribs have degenerated. 



LEPTOPECTEN Verrill, 1897 



!.i'l>ti>i>'tni (sul)-ciiiis of Cli lam i/x) VKKUILI., Trans. Conn. Ai-:iil.. X, jj>. ('>!, S!i. Ill, 

 IS! IT. 



n im.nuitiiiii-rix i. Conrad). 

 Shell thin, translucent, oblique, broadly rounded, with strong, 

 rounded radial ridges or folds, like corrugations, which appear in 

 reverse on the interior surface. The internal ribs are not angulatedby 

 a deposit of shell, nor distinctly thickened. Margin with broad scal- 

 lops. The exterior surface is covered with tine divergent camptonectes 

 sculpture, both on the ribs and intervals. The ribs do not increase in 

 number with age but become broader and more flattened. Auricles 

 large and broad, thin, corrugated. Byssal notch large and deep. Pec- 

 tinidial teeth prominent. Hinge-plate thin and but little differentiated. 

 Cardinal ridge thin and small, close to the ligament, crossed by fine 

 incisions. 



PLACOPECTEN Yen-ill, 1897. 



rt<i<-<>i>-i<n (suliiifiiiis of Clilamys) VERRULL, Trans. Conn. Acad., X, pp. 69, 89,91, 

 ]>1. \YIII. tins. 1-7: i)l. xx, tigs. 7,8, 8a; pi. xxi, figs. l-2a, 



Type. ridrojicctcii cl intoning (Say). 



Shell large, compressed, broadly rounded, rather thin, with simple 

 sharp edges, meeting evenly ventrally, but gaping considerably at 

 both ends, especially when adult. Valves only slightly unequal in 

 form, the right one being a little flatter, but they differ in color and 

 somewhat in sculpture, the right one being smoother and paler. Both 

 have tine radial lines or riblets, and they have vermiculated divergent 

 riblets when young. Auricles small, symmetrical, nearly equal. 

 Byssal notch small, simple. Pectinidial teeth generally obsolete, except 

 when young. No internal ribs. Inner surface often with more or less 

 pearly luster and a crystalline structure. Hinge-plate with two feeble. 

 slightly divergent ribs on each end, crossed by fine transverse inci- 

 sions. The foot ' is well developed, oblique, slightly narrowed distally 

 and enlarged at the end, where it is divided into two lobes by a rather 

 deep, oblique, longitudinal fissure, so that the lobes can be spread 

 apart or closed at will, thus resembling somewhat the foot of Le<li<I'. 

 Toward the base, on the anterior side, there is also a short, deep byssal 

 slit, terminating at a prominent tubercle about the middle of the front 

 side. 



1 Trans. Conu. Acad., X, pi. xx, fig. 8. 



