SCROBICULARIA. 435 



A. Lateral teeth more or less developed. 



H 

 1. SCROBICULARIA PRISMA%CA,*, (MontagU) 



Ligula prismatica, Mont. Test. Brit. Suppl. p. 23, t. 26. f. 3. Syndosmya 

 prismatica, F. & H. i. p. 321, pi. xvii. f. 15. 



BODY pearl-white : mantle fringed with short and numerous 

 cilia: tubes cylindrical and slender, indistinctly annular or 

 corrugated by the investing sheaths ; they are nearly of the 

 same length, the incurrent being the larger or wider but 

 usually the shorter of the two ; excurrent or upper tube stran- 

 gulated or constricted at irregular intervals ; both tubes are 

 clothed with a few scattered papilla?, and the orifice of each 

 has from 5 to 7 short cirral points, which are sometimes obso- 

 lete or imperceptible : gills two on each side, suboval, of equal 

 size, distinctly pectinated, faintly tinted with brown : palps 

 the same in number, thin, short, broad, and angular, smooth 

 without and striated within : foot large and pointed. 



SHELL oblong and wedge-shaped, scarcely convex, but not 

 flattened except towards the posterior end, very thin and fra- 

 gile, semitransparent, highly polished and glossy: sculpture, 

 numerous slight, irregular, and minute concentric stria?, which 

 occasionally are laminar or scratch-like, and towards the mar- 

 gins become partially oblique, as in Tellina fdbula ; the lines 

 of growth are also more or less conspicuous: colour pearj- 

 white : epidermis a mere film, reflecting in certain lights a pris- 

 matic lustre : margins curved in front, semioval or rounded on 

 the anterior side, and bluntly pointed on the posterior side, 

 which is flexuous and obtusely angulated; dorsal margin nearly 

 straight on the anterior side, incurved below the beak, sloping 

 outwards from the posterior angle, and obliquely truncate on 

 that side, the point or terminal angle lying in the transverse 

 axis of the shell : beaks very small ; umbones somewhat pro- 

 jecting: ligament short, prominent, yellowish-brown: carti- 

 lage curved, horncolour, proceeding from a narrow chink under 

 the beak, and expanding in a shelving and oblique direction 

 below the hinge-plate on the posterior side: hinge-line ob- 

 tusely angular and flexuous: hinge-plate rather thick, much 

 broader and reflected behind the cartilage : teeth, in the right 

 valve two minute, erect, equal-sized, parallel, and obliquely 

 set cardinals, besides a long but slight lateral on each side, 



* Having a prismatic lustre. 



