87 



Longitudinal, projecting, radiating ribs, with or with- 

 out transverse stria. 

 Pectunculus castaneus Pectunculus pectinatus 



pectiniformis radians 



Pectunculus vitreus. 

 [There are also nine fossil species.] 



NUCULA. 

 arca pella. — Linn. oH "% 



Chetn. 7, tab. 53, fig. 550, 55 1 . * ■ 



This genus appears to form an evident transition to th 

 G. Trigonia, and is the last division LaMarck has made 

 of the genus Area of Linnaeus; from which, and the two 

 other divisions, it differs, particularly in consequence 

 of the ligament being partially inserted internally, with 

 out the angular groove on the exterior of the valves. 

 Shell small, subtriangular, ovate, striated transversely, 

 somewhat beaked at the anterior end : apices contigu- 

 ous, and curved backward ; ligament marginal, partly 

 internal, no angular channel. Hinge linear, many 

 teeth, with an oblique projecting spoon-shaped cavity 

 in the centre of it, to which the ligament is attached. 



