254 LUCINID.E. 



Diplodonta rotunda'ta*, Montagu. 



Tellina rotundata, Mont. Test. Brit. p. 71, tab. 2. f. 3. D. rotundata, 

 F. & H. ii. p. 66, pi. xxxv. f. 6, and (animal) pi. M. f. 7. 



Body clear white : gills laminar, somewhat elongated and 

 angular, folded or puckered at the edges, pectinated, and fixed 

 as usual to the dorsal range, the upper plate being little more 

 than half the depth of the lower one : palps short, pointed, 

 pectinated, and somewhat triangular : orifice at the posterior 

 end very small and inconspicuous : foot central, rather long, 

 and conical, minutely perforated at the point. 



Shell obliquely quadrilateral, gibbous, not very thin, opaque, 

 moderately glossy: sculpture, strongly marked and irregular 

 lines of growth, and (under the microscope) exceedingly fine 

 and close-set, regular longitudinal lines on the umbonal area, 

 which radiate from the beak and are only perceptible in fresh 

 specimens, as well as equally fine and numerous, but irre- 

 gular and often wavy or confluent, transverse lines on the 

 rest of the surface : colour milk-white : epidermis fibrous and 

 yellowish, usually rubbed off except in front : margins obtusely 

 angular on the anterior side, where the slope to the beak and 

 front is nearly equal, obliquely rounded in front, somewhat 

 produced and very much broader on the posterior side, which 

 is also rounded at the end, and straight behind : beaks small, 

 scarcely projecting beyond the dorsal line, and not contiguous : 

 lunule very small, but deep, not well defined, and hidden by the 

 beaks : ligament short, semicylindrical, yellowish-brown or of 

 a golden hue, projecting beyond the dorsal margin ; ligamental 

 groove rather deeply cut : hinge-line slightly curved, occupy- 

 ing nearly one-third of the circumference : hinge-plate broad 

 in the middle and tapering gradually to each end ; the pos- 

 terior side is the longer: teeth, in the right valve two erect 

 cardinals, the posterior of which is double and placed more 

 obliquely than the other, as well as a plate-like lateral on each 

 side, the anterior of which is distinct ; in the left valve the 

 same number and kind of cardinals and laterals, but the posi- 

 tion of the former is reversed : inside marked by slight longi- 

 tudinal lines, frost-like in the centre and at the back, but 

 glossy towards the margin, which is plain : pallial scar distinct, 

 broad, and entire : muscular scars large and well marked, the 

 anterior one irregularly elliptical, and the posterior oval. 

 L. 0-9. B. 1. 



* Rounded. 



