EEPORT ON THE LAMELLIBKANC'HIATA. 207 



brevissimus, infra umbones leviter concave obliquus, posticus elongatus, primo rcctiusculus, 

 fere horizontalis. Umbones parvi, acuti, antrorsum iuclinantes, antemediani, circiter in 

 ^ longitudinis coUocati. Valva sinistra dente unico ereeto lamelliformi utrinquc nnmita, 

 dextra marginibus dorsalibus lamelliformibus subdentata. Ligamentum intormim iufra 

 umbones situm. 



A single specimen is all that is at present knowTi of this species. It is very small, 

 moderately convex, ovate, rather narrower in front than behind, very inequilateral, thin, 

 white, glossy, and sculptured with excessively fine lines of growth. The front dorsal 

 margin is short, a little oblique, just a trifle excavated before the umbones, then some- 

 what arcuate and gradually rounding into the sharply curved extremity. The posterior 

 margin is much longer, at first almost horizontal and straight, afterwards becoming a 

 little arcuate and oblique. The ventral outline is widely curved and equally ascending 

 at both ends. The beaks are small, curved over towards the front, very slightly 

 produced, and situated in advance of the middle, at about one-third of the total length 

 from the anterior end. The left valve has two erect lamellar very divergent teeth, of 

 which the posterior is rather more strongly developed. Tliey are separated by a broad 

 triangular space, and stand out free from the outer margin. The right valve has no 

 distinct teeth, but has the dorsal margin on each side elevated into thin prominences, 

 which when the valves are closed fit into the grooves between the teeth and dorsal margins 

 of the other valves. The ligament is internal, and situated beneath the umbones. All 

 the muscular impressions on the inner glossy surface ai*e indistinct. 



Length 2^ mm., height if, diameter Ij. 



Habitat. — Station 24, ofi"Culebra Island, West Indies, in 390 fathoms ; Pteropod ooze. 



It is impossible to say whether the above figures represent the full size attained by 

 this species, but probably they do not. 



Montacuta pura, n. sp. (PI. XII. figs. 6-66). 



Testa elongata, valde insequilateralis, tenuissima, alba, incrementi lineis tenuibus 

 striata, utrinque rotundata, postice paulo angustata. Margo dorsi anticus elongatus, 

 levissime arcuatus, fere horizontalis, posticus brevior, oblique curvatus. Margo inferior 

 latissime curvatus. Umbones parvi, paulo supra marginem product!, ad apicem obtusi, 

 longe pone medium coUocati. Dens cardinalis unicus in valva dextra infra apicem situs, 

 in sinistra nullus. 



This little species is remarkably oblong, very fragile, white, very inequilateral, 

 obtusely rounded at the anterior end, and a little narrower at the opposite extremity. 

 It is smooth, glossy, rather convex, and sculptured only by the very fine concentric lines 



