MALACOZOA. TROPIOPODA. LAMELLIBRANCHIATA. 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 submarginal 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 esophagus 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. 



