REPORT ON THE LAMELLIBRANCHIATA. 75 



having the apices rolled up remote from one anotlier and the hinge-line, and situated quiti- 

 near the anterior end. The posterior dorsal margin is rather straight, onl}'' a trifle 

 oblique, and joins the broadly arcuate lateral outline in a well-rounded curve. The 

 anterior portion of the upper mai-gin is very short, forming merely a rounded angle with 

 the obliquely truncate, almost rectilinear, anterior side. The ventral outline is semicircu- 

 larly curved, and more gently ascending posteriorly than in front. The interior is white, 

 glossy, and apparently more or less iridescent. The anterior muscular impression is 

 elongate, of an irregular form and high up, just beneath the umbones. Behind there are 

 three impressions, one (the largest of all) is situated about the middle of the posterior side, 

 the two others being under the dorsal margin. The hinge-line is simple and toothless, 

 having a thin external ligament attached to the edge along nearly the whole extent. The 

 internal Ligament is very slender, located in a very narrow groove, near, but within tlic 

 edge, and probably supported by a free ossicle. 



Length 14 mm., height 16, diameter 12. 



Habitat. — Station 133, Mid South Atlantic, in 1900 fathoms; Globigerina ooze. 



Only a single dead specimen of this fine species was obtained. It differs from 

 Lyonsiella papyracea, not only in form and smaller size, but also in having much more 

 numerous lirse. 



Silenia, n. gen. 



Testa tenuis, ovato-oblonga, iutus margaritacea, concentrice striata, sparsini 

 granulata; cardo edentulus ; ligamentum externum, marginale. Animal ovato-oblongum, 

 convexiusculum ; pallium inferne apertum, postice siphones infra clausum. Apertura 

 siphonalis papillis elongatis couicis scabris circumdata ; sipho branchialis magnus, externe 

 hand prominens, intus in tulja magna productus, anali contra minimo, externe paulo 

 exstanti, intus baud prolongate ; pes parvus, gracilis, sulco byssali baud aratus. Papillae 

 oi'ales inajquales ; anticse permagnse, posticse parvse. Branchite insignes, utrinque in s(M"ie 

 interrupta, posticeque serie altera transversa etiam in medio interrupta digestae. 



This remarkable form differs from Lyonsiella in possessing well-developed labial palpi, 

 in the character and disposition of the branchiae, and one or two points with regard to the 

 siphonal opening and the foot, &c. The shell may be distinguished from that of Lyonsiella 

 by the difference in the hinge-characters, the latter genus having an internal ligament 

 supported by a free ossicle, whilst in Silenia it is external and without a shelly support. 



Sileriia sarsii, n. sp. (PL XXV. figs. 4-46). 



Habitat. — Station 157, about 1100 miles south-west of Australia, in 1950 fathoms; 

 Diatom ooze; also Station 325, east of the mouth of the Rio de la Plata, in 2650 fathoms: 

 blue mud. 



