PECTUNCULUS. 165 



shaped when at rest like an axe, bnt capable when in motion 

 of being expanded into a discoidal form. 



Shell suborbicular, convex, nearly equal-sided, porcel- 

 lanous : epidermis velvety : beaks slightly incurved, and be- 

 coming separated from each other in the course of growth. 

 ligament altogether external, very strong, occupying a vaulted 

 cavity at the back of the shell, composed of several bundles of 

 cylindrical fibres, which radiate from the beak to the hinge- 

 plate, to the outer edge of which they are united: teeth 

 laminar and continuous, arranged in a curved line but in two 

 distinct rows : pallial scar entire : muscular scars oval, sym- 

 metrical, and strongly marked. 



The rounded form of Pectunculus prevents its being 

 mistaken for any other genus of the same family, except 

 Limopsis ; and the substitution of a compound ligament 

 for a simple cartilage, besides other specialties of the 

 hinge-structure, offer sufficient marks by which these 

 genera can be known one from the other. Moreover 

 Pectunculus has never been observed to produce a byssus : 

 but I do not consider this a distinguishing characteristic. 

 The nature of their habitation is the same. 



The present genus has descended in an unbroken line 

 from the Silurian epoch to the present time. The 

 extent of its distribution in space is equally great. It 

 is prolific in species ; but only one of them has yet been 

 found as far north as the Loffoden Isles. This is the 

 kind that inhabits our seas. 



Lister was the first naturalist, since Pliny, to use the 

 word Pectunculus; but he applied it to most bivalve 

 shells. His second division of Pectunculi had a fright- 

 fully long adjective — Polyleptoginglymi — and comprised 

 Area and Pectunculus. Eight years before Lamarck 

 published the present genus, Poli proposed the name 

 Axincea for the animal. This last name has been adopted 

 by Oken and Gray ; but as it was founded solely on 

 anatomical, and therefore insufficient characters, it does 



