72 THE VOYAGE OF H.M.S. CHALLENGER. 



This very interesting form is rather compressed, fragile, hroadly ovate, rounded in 

 front, narrower and somewhat pouting posteriorly. It is very inequilateral, minutely 

 granulated, striated by concentric lines of growth, dirty whitish, exhibiting traces of a 

 pale earth-coloured epidermis, which is somewhat reflexed within the valves. The valves 

 are about equal in size, bent a little to the right at the posterior end, and gape somewhat 

 at ])oth extremities, especially behind. The front dorsal margin is slightly convex, very 

 long, and only a little oblique, the posterior being very much .shorter and rather straight 

 at first, then, descending in a slight curve, joins the obliqi;ely up-curving and .slightly 

 sinuated ventral margin at an obtuse point, which is most conspicuous in the left valve. 

 The umbones are small, acute, slit at the apex, and situated at about one-third of the 

 entire length from the hinder end. The cartilage-receiver is more or less ovate, directed 

 obliquely towards the anterior ventral edge, and supported liy a shelly ridge. The 

 transverse free ossicle is moderately thick. The interior of the valves is whitish, a Uttle 

 pearly, exhibits the concentric lines of growth, and one or more radiating lines from the 

 beaks down the posterior side, and apparently more conspicuous in the left valve than 

 the right. The pallial sinus is moderately deep, reaching rather more than one-third 

 across the valves, and is sharply rounded at the end. 



Length 37 mm., height 28, diameter 13. 



Habitat. — Station 321, off Monte Video, in 13 fathoms ; mud. 



The types described by d'Orbigny were collected a little farther south, near San Bias, 

 in North Patagonia. 



Lyonsia, Turton. 



Lyonsia formosa, Jeffreys (PL YI. figs. 3—36). 



Lyonsia funnnsa, Jeffreys, Proc. Zool. Soc. LonJ., 1881, p. 93l), [il. Ixx. fig. 1. 



Habitat. — Station 8, off Gomera, Canaries, in 620 fathoms ; sandy mud and 

 shells. (For further distribution, see Jeffreys, loc. cit.) 



The Challenger specimen is oblong and somewhat quadrate, having two distinct ridges 

 radiating from the umbones, one to the ventral margin at a point a little l)ehind the 

 middle, and the other to the posterior lower extremity, the former being rounded and 

 subnodose and the latter even, rather acute, and bearing a series of granules rather larger 

 than those on the rest of the surface. Both ends are gently curved, and the lower 

 outline is a little sinuated on each side of the subcentral carina. Behind the posterior 

 keel the valves exhibit about seven slender feeble radiating ridges, each with a row of 

 granules along the top, which are similar to those upon the carina. Within the valves 

 there are seven corresponding faint furrows which have a subpunctate appearance. 



