ARCA. 235 



Area navicularis, Desh. (not Brug.) Anim. s. Vert. (ed. Desh.) vol. vi. p. 462. 

 — Philippi, Moll. Sicil. vol. ii. p. 42. 

 „ Briiannica, Reeve, Conch. Icon. Area, pi. 15, f. 98. 

 „ rhombea, Couch, Cornish Fauna, pt. 2, p. 31. 



It is but i-arely that we meet with this Area in fine and 

 perfect condition ; much more frequently it occurs only in 

 single valves, or, from the nature of its dwelling-place, with 

 a considerable portion of its surface abraded. In this last 

 state it has been separated by Turton from its more typical 

 form (the fusca of Donovan) under the name tetragona ; 

 whilst aged and worn examples have been confounded with 

 the true Noce by Montagu and Donovan. 



Its valves, which, although broad across the umbones, 

 where the surface is much flattened, rapidly diminish in 

 convexity, are of an elongated subrhombic shape, strong 

 and opaque, devoid of lustre, and of equal size and pro- 

 fundity. They are of a warm rufous brown or reddish 

 chocolate colour, with the colouring matter not evenly 

 diffused, but wholly or partially absent near the front of 

 the shell, and disposed in darker concentric zones upon the 

 hinder portion. In the more beautiful specimens the ante- 

 rior third is almost white, with, however, a more or less 

 distinct radiating brown stain in front. The surface is 

 everywhere adorned with most closely arranged radiating 

 costellae, which are decussated by still more closely-set 

 concentric and somewhat imbricated granules: the inter- 

 stices of the costellse, which are of equal size throughout 

 the entire exterior (the few, however, which immediately 

 follow the umbonal keel become changed into radiating 

 stria)), are so narrow, except near their termination, as 

 almost to be linear, and are clothed with a brownish and 

 somewhat squamular epidermis. The umbonal ridge is 

 sharply angular, and upon its crest the decussation becomes 

 almost squamular, and the epidermis of an increased 



