CARDIUM. 295 



ceptible by the naked eye : colour yellowish-white, beautifully 

 mottled in the young and near the beaks of the adult with 

 blotches or longitudinal streaks of reddish-brown or fleshcolour : 

 epidermis thin, consisting of a tissue of exceedingly numerous 

 and delicate fibres, woven in a transverse direction, frequently 

 anastomosing or becoming blended together, and varying in 

 colour from yellow to brown : marc/ins curved in front and 

 at the sides, with a slight tendency to form an angle at the 

 posterior extremity ; the rounded contour is only interrupted 

 behind by the prominence of the umbonal area : beaks small, 

 almost touching each other, slightly recurved towards the 

 anterior side : tunule indistinct, but having projecting lips : 

 Ugamentlojig, spindle-shaped, horncolour, and continued under 

 the beaks on the anterior side : hinge-line obtuse-angled : 

 hinge-plate broad, reflected outwards, not occupying one- 

 fourth of the circumference : teeth, in the right valve two car- 

 dinals, a strong triangular lateral on the anterior side and a 

 smaller pointed lateral on the posterior side ; in the left valve 

 are also two cardinals, two laterals (the lower one being very 

 much larger than the other) on the anterior side and one 

 short triangular lateral on the posterior side ; the cardinals are 

 relatively of the same size in each valve : inside porcellanous, 

 closely and microscopically striated like the epidermis, some- 

 times of a chocolate colour, or variegated with reddish-brown ; 

 margin fluted in the middle and towards the sides, but only 

 notched near the hinge-line : pallial scar distinct : muscidar 

 scars oval and deep. L. 2-4. B. 2*3. 



Var. 1. gibba. Shell smaller, having a more oblique con- 

 tour, and gibbous. 



Var. 2. rotunda. Shell flatter and rounder. 



Var. 3. pallida. Shell of a uniform strawcolour. 



Habitat : Generally, but locally, diffused throughout 

 the British seas, on sandy and nullipore bottoms, in 5- 

 80 fathoms. Var. 1. Exmouth (Clark); Swansea and 

 Bantry Bay (J. G. J.); Milford Haven (Jordan). Var. 2. 

 Birterbny Bay, Connemara (Barlee) . Var. 3. Exmouth 

 (Clark); Sark (J. G. J.). This species is a member of 

 our more recent pliocene beds, as well as of the Coral- 

 line Crag. Mr. Rose has found a specimen in a stratum 



