KEPORT ON THE LAMELLJBRANC}II AIA. 77 



umlionibus ii.s(jue ml cxtrcmitatem iuferioicni postieam arata, radio parum elevato 

 iimboues infra subperpeudiculari interdum vix conspicuo instructa. JVIargo dorsi anticus 

 obliquus, leviter arcuatus, posticus longior, liorizoutalis, prope umbones concavus. 

 Ventris marge rectiusculus. Latus antieum l)revis, subacute rotundatum, vix hians, 

 posticum latum, apertissimum. Umbones parvi, acuti, ante medium siti. Cartilage 

 interna inter callos duos minutos posita ; pagina interna uitida, plus minusve prismatica, 

 superne infra umbones porca obli(|ua gracili, tubcrculo juncta, margine minute dentate 

 instructa. 



This shell is oblong, half as long again as high, very thin and fragile, semitransparent 

 white, concentrically rather coarsely wrinkled and striated, exhibiting on each valve a 

 shallow groove radiating from the umbones to the lower posterior extremity, and towards 

 the front having (but not always) a faintly elevated ray, which falls almost perpendicularly 

 beneath the beaks to the ventral margin. It is rather inequilateral, much gaping, broad 

 and arcuately truncated posteriorly, uarrow^ed and somew^hat sharply curved in front, and 

 straightish along the lower outline. The anterior dorsal slope is obliquely descending 

 and faintly convex, the posterior being a little concave near the beaks, and then 

 horizontal or even a trifle ascendant. The umbones are small, acute, and h cated 

 rather in front of the centre. The hinge-cartilage is placed between two minute 

 shelly processes from which diverge in an oblique dii'ection towards the hinder extremity 

 of each valve a slender elevated ridge, the edge of which, under the microscope, is 

 seen to be minutely serrated or dentate. The muscular scars and mantle-impression are 

 indistinct. 



Length 12 mm., height 8, diameter 5. 



Habitat. — Betsy Cove, Kerguelen Island, in 15 to 25 fathoms, January 9, 1874 ; 

 and Eoyal Sound, Kerguelen Island, in 28 fathoms. 



A specimen of this species, more than two inches in length, was collected at Cumber- 

 hind Bay, Kerguelen Island, during Captain Eoss' voyage, and is now in the British 

 jMuseum. In the same collection are two or three others from New South Shetland, 

 (me presented by Captain P. P. King, E.N., being the type described by himself in 

 conjunction with Mr. Broderip. In the adult shell, which is rather solid and beautifully 

 pearly under the external calcareous surface, the internal septum loses its denticulate 

 character. 



The muscular scars are high up near the dorsal margin, both being ef an elongate 

 form. The sinus in the pallial line is very wide, and varies somewhat in depth in 

 different specimens. The adult shells show considerable variation in the relative length 

 of the two sides. Some specimens are almost equilateral, whilst others have the anterior 

 portion longer than the posterior, the reverse l)eing the case with the young individuals. 

 The figure given by Eeeve gives a i-ather unusual up-curving of the hinder dorsal margin, 



