PECTUNCULUS. 



ment is attached: this external area, with its angular 

 grooves, distinguishes the Pectunculi, as well as all the other 

 Arcacece from Nucula, which has an internal ligament, and 

 no area between the umbones, consequently should not be 

 arranged with the Arcacece : to which it bears no resem- 

 blance but in the number of its teeth. 



Shell orbicular, sublenticular, equivalve, subequila- 

 teral, shut close. Hinge teeth numerous, oblique, placed 

 in two rows, one on each side of the umbo, forming a 

 curved or arched line : alternately inserted: those nearest 

 the umbo nearly obsolete. Ligament external, attached to 

 an area on each valve, between the umbones. Muscular 

 impressions two, lateral, distant. 



The Pectunculi are marine shells, and have undoubt- 

 edly been so named from their resemblance to the Pectines, 

 in form, and in their internally crenulated edges, as well 

 as their longitudinal rays. They are covered with a 

 velvety epidermis, except about the umbones, where it is 

 generally worn olF. The species are not very numerous, 

 nor are they easily distinguished; the P.pilosus (Area 

 pilosa. Linn. J is not unfrequent upon the southern coasts 

 of our Island, it is one of the prettiest species, particularly 

 when young, and belongs to Lamarck's first section, with 

 slight longitudinal grooves : his other section, with strong 

 radiating longitudinal costce or ribs, contains but few extra- 

 Europaean species. All the fossil species we know, are 

 found either in the London Clay or the Calcaire Grossiere : 

 a very neat one occurs in the sandy indurated Marie, be- 

 longing to the London Clay at Bognor, where it is accom- 

 panied by fossil Pinnae, Lingulae, &c. We have represented 

 this, believing it fo be a variety of Lamarck's Pectunculus^ 

 pulvinatus^ but, in truth, it is so difficult to fix the characters 

 of the species in this Genus, that we dare not speak decid- 

 edly upon this point. 



Fig. 1. Is Pectunculus pilosus, of which we have given the outside of one, and 

 the inside of both valves, to show the hinge. 



