REPORT ON THE LAMELLIBRANCHIATA. 31 



posticam decurrexitem tenuioribus, umbonesque versus subobsoletis. * Margo dorsi antifus 

 valde obli(|uus, subcoucavus, posticus minus declivis, fere rectilinearis, elongatus. 

 Margo ventris in medio late sinuatus, utrinque leviter excurvatus. Umbones parum 

 prominentes, albi vel rubeseentes, Isevigati. Pagina interna alba, plus minusve olivaceo- 

 fusco maculata, margine exterior! valvse sinistrse in testis adultis liic iUie minute denti- 

 oulato. Dens cardinis valvse dextrte erectus, aeutus. 



This is a large species, very long, not particularly convex, whitish, only slightly 

 inequivalve, but very inequilateral. It is nearly twice as long as high, rounded at the 

 anterior end, obliquely and sharply truncated behind, terminating in an acute point. 

 The valves are moderately thick, and divided into two unequal parts l)y a sharp keel 

 ranning obliquely from the beaks to the hinder extremity. They also exhibit a shallow 

 depression down the middle, causing a sinus or incurvation in the ventral margin, which 

 otherwise is slightly arcuate. The dorsal slopes are unequal, the anterior being short, very 

 oblique and faintly concave, the posterior longer, less descending and nearly rectilinear. 

 The sculpture (consists of moderately strong, concentric ridges, which gradually increase 

 in thickness with, the growth of the shell, becoming behind the radiating carina slighter 

 than in front. In addition, the entire surface exhibits very numerous, excessively fine, 

 radiating lines, which are minutely granular, especially conspicuous in the grooves 

 between the costae. The beaks are only a little prominent, curve over towards the front, 

 and are almost smooth at the tips, which are usually white, but at times pinkish. The 

 single cardinal tooth of the right valve is erect, curved, and acute. The interior of the 

 valves may be either almost totally white, or more or less stained with olive-brown, or 

 occasionally with a pinkish tint. The margin of the left valve in old specimens is 

 more or less minutely denticulated aU round, a corresponding groove just ^\ithin the 

 circumference of the right valve being minutely pitted. 



Length 26 mm., height 13, diameter 9. 



Habitat. — Station 188, south of New Guinea, in 28 fathoms; bottom, green mud. 



Young shells of this species and of Corhula scaphoides are very much alike. The 

 latter are, however, flatter, more inequilateral, and have fewer and less elevated ridges, 

 which are much more obsolete behind the oblique keel. 



This species is not likely to be mistaken for any other on account of its large size and 

 very elongate form. Corhula crassa offers some resemblance, but is proportionally 

 higher, more ec^uilateral, thicker, more coarsely ridged towards the umbones, more 

 suddenly contracted at the posterior or rostrated extremity, and has stronger hinge 

 characters. 



