MALACOZOA. TROPIOPODA. LAMELLIBBANCHIATA. 287 



lique triangular fossa, for the ligament; lateral teeth 

 elongated, thin, crested, with a groove between them 

 and the margin ; the right valve similar, but with the 

 plicate cardinal tooth nearer the umbo, and an additional 

 thin lamina in each of the lateral sub marginal grooves. 

 Internal ligament trigonal, rather large ; external small, 

 close to the umbones. Muscular impressions rather 

 large, oval, submarginal ; pallial impression with a sinus 

 behind. 



1. Mdctra stultorum. Simpleton's Mactra. 



Shell ovato-trigonal, thin, subdiaphanous, with delicate ob- 

 solete concentric striae, more prominent on the posterior slope ; 

 the internal surface glossy ; umbones purplish-red, the general 

 colour light yellowish-red or carneous, radiated with lines and 

 bands of dull white ; the posterior slope paler, with a lanceolate 

 dull-red medial patch ; inner surface of a delicate bluish-pur- 

 ple tint. Length two inches, height an inch and a-half ; the 

 largest I have met with two inches two-twelfths, an inch and 

 eight-twelfths. 



It varies in its markings, and is sometimes of a nearly uni- 

 form yellowish-grey. Old individuals frequently have the 

 epidermis rather thick toward the margin. 



The animal elliptical, compressed ; with the mantle ex- 

 tremely delicate, but thickened toward the margins, which are 

 simple, united anteriorly, and posteriorly, in the latter situa- 

 tion forming a short, compressed siphon, of two tubes, the 

 lower larger for respiration. The branchiae large, nearly 

 equal, very thin, striolate. The mass of the body enclosed in 

 a muscular envelope, thin at the sides, thick anteriorly and 

 posteriorly, and continuous with the foot ; which is very large, 

 bent forward, much compressed, tapering, thin-edged. At 

 the anterior part of the body is the small, cylindrical mouth, 

 with four long, compressed, tapering tentacular appendages : 

 the oesophagus short, cylindrical ; the stomach moderate, with 

 a long tapering ccecal appendage; the intestine convoluted 

 round the liver, then ascending to near the hinge, passing 

 through the heart, and terminating in the upper siphonal aper- 

 ture. The liver forms a very large mass of a brownish colour, 

 occupying half the body anteriorly. The heart oblong, si- 

 tuated under the hinge. Over the mouth is a small ganglion, 

 and at the commencement of the rectum a large nervous mass. 



