ARCA. 169 



ventral spines of certain Echinoderms. Living speci- 

 mens of P. glycymeris, which I have dredged in 85 

 fathoms, had their shells beautifully marked by varie- 

 gated streaks of a bright reddish-brown. 



Mr. Searles Wood has taken a great deal of pains in 

 making out the synonymy of this variable species in a 

 recent and fossil state. He cites no less than eighteen 

 different names. Among the best-known of these are 

 Area pilosa (Linne), A. bimaculata (Poli), Pectuncidus 

 stellatus (Lamarck), P. undatus, decussatus, and num- 

 marius (Turton, but not Linnets species of Area bearing 

 these names), and P. lineatus (Philippi). The Area 

 minima of Turton' s ' Conchological Dictionary ' was ad- 

 mitted by him, in his ' Dithyra/ to be the fry of this 

 species, although Leach subsequently referred it to A. 

 Noes. Mr. Hanlev at first asserted that, from an exa- 

 mination of Linnets own specimens, his A. glycymeris 

 was the P." violas cens" (violas cescens) of Lamarck ; but 

 he afterwards corrected the mistake. The first localitv 

 given by Linne (on the authority of Lister) for A. gly- 

 cymeris was " Garnsey," where P. violascescens has never 

 been found. The last-named species appears to be his 

 A. nummaria, judging from the description in the ( Sys- 

 tema Naturae/ 



Genus V. ARCA*, Linne. PL IV. f. 5. 



Body oblong and thick : mantle entirely open, except at the 

 back, in some species fringed with tentacular filaments, or 

 furnished (as in Pectuncidus) with ocelli : foot large and ex- 

 tensile : byssus composed of glutinous threads, which some- 

 times form a compact mass, or plug of attachment. 



Shell oblong or rhomboidal, gibbous, inequilateral, and in 

 a few species slightly inequivalve : epidermis fibrous : ligament 



* A chest ; or from the supposed resemblance of Noah's ark to this shell. 



I 



