LTJTRARIA. 429 



Shell elliptical, compressed, moderately solid, and glossy : 

 sculpture, irregular and slight concentric striae alternating with 

 stronger lines of growth : colour yellowish -white under the 

 ejptdermis, which is olivaceous, glossy and like oilskin, wrinkled 

 or puckered, delicately fibrous at the sides and in front, and 

 minutely striated on the umbonal part in a longitudinally 

 radiating direction : margins nearly straight or slightly in- 

 dented and flexuous in front, and sloping obliquely to each 

 side, the anterior being rounded or semioval and the posterior 

 bluntly angulated ; dorsal margins scarcely (if at all) curved, 

 inclining forwards, and diverging from an obtuse angle : beaks 

 small and sharp, incurved; umbones not much projecting: 

 ligament small and short, greenish-brown : cartilage triangular 

 and thick, narrower in the middle and spreading out at each 

 end in the hinge-cavity, yellowish-horncolour : hinge-line 

 nearly straight : hinge-plate long and strong, projecting in the 

 middle to form a support for the cartilage : teeth, in the right 

 valve two diverging cardinals on the anterior side, and a short 

 slight and laminar lateral on the posterior side of the cartilage; 

 in the left valve an erect and large double cardinal immedi- 

 ately below the beak, flanked by a smaller but pointed one 

 (like a leaf) on the posterior side, and by a sharp triangular 

 lateral on the anterior side, besides a slight laminar lateral on 

 the posterior side, corresponding with that of the opposite 

 valve : inside porcelain-white, polished and iridescent in the 

 scar-marks, and microscopically wrinkled or frost-like in every 

 other part : pallial scar double, with an oblong sinus extend- 

 ing about two-thirds of the distance across : muscular sears 

 deep, placed high up near the dorsal margins, anterior pear- 

 shaped, posterior triangularly oval. L. 3. B. 5*5. 



Var. alterutra. Shell smaller, thicker, and proportionally 

 broader ; front and back margins nearly parallel, anterior side 

 obliquely truncate. 



Habitat : Every part of our seas, from the lowest 

 verge of spring-tides to 15 fathoms, in soft or slushy 

 sand. The reason of this species being chiefly found in 

 estuaries and at the mouths of rivers appears to be 

 owing to an accumulation of mud being washed down 

 from the land and deposited outside the line of mean 

 tide-marks, which suits the Lutraria as a burrowing-bed. 

 The variety has been found by Professor King on the 



