SAXICAVA. 77 



The evidence that P. glycimeris has been found in 

 our seas is not satisfactory; this species inhabits the 

 Lusitanian and Mediterranean coasts. 



Genus II. SAXI'CAVA*, Fleurian de Bellevue. 



PI. III. f. 3. 



Body muscular : tubes extensile, diverging at their extremi- 

 ties, and covered by a leathery or membranous sheath : gills 

 prolonged into the cavity of the branchial tube : foot furnished 

 with a byssal groove. 



Shell often inequivalve, wrinkled transversely, gaping at 

 the posterior end, and sometimes also in front (or what may 

 be termed the antico -ventral part) : epidermis thick : ligament 

 short, prominent, attached to an elongated process of the 

 hinge-plate : teeth, a small conical cardinal in the right valve, 

 locking between two similar ones in the left, but frequently 

 obsolete or wanting : pallial sear interrupted or broken up 

 into separate spots, not deeply sinuated. 



The doubtful position which this genus formerly occu- 

 pied among bivalve shells appears from the circumstance 

 that Linne called the typical species (S. rugosa) and its 

 variety arctica respectively Mytilus and Solen, Fabricius 

 Mya, Strom Chama, Poli Donax, Solancler Venus, Bru- 

 guiere Cardita, and Turton Anatina ; and that the variety 

 constituted the genera Hiatella of Daudin, Clot ho of Faujas 

 St. Fond, Byssomya of Cuvier, Byssonia, Rhombus, and 

 Rhomboides of De Blainville, Didonta of Schumacher, 

 Biapholius, Coramya, and Pholeobia of Leach, and Agina 

 of Turton. Grav makes Hiatella and Saxicava distinct 

 genera. The former name was published in 1799, and 

 the latter in 1802 ; but Daudin did not sufficiently 

 characterize his genus, and Saxicava may be considered 

 as now established by general usage. According to 

 Chenu the geological age of the present genus dates from 



* Rock-perforator. 



