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



This fiue species is excessively thin, globose, roundly heart-shaped, inequilateral, 

 transparent wliite, but rendered rather opaque by a thin dirty-white epidermis. The 

 sculpture consists of numerous (about forty) very slender hair-like lines, which radiate 

 from the beaks to the outer margin. They are fairly equidistant, and cover the whole 

 surface with the exception of a small space in front of the umbones. The valves are also 

 very finely striated by the lines of gi'owth, a few at intervals, which apparently mark 

 periods of arrested enlargement, being quite conspicuous. The umbones are large, 

 prominent, well incurved, situated considerably in front of the centre, and inclined over 

 towards the anterior side. The hinge-margin is very thin, and absolutely toothless. 

 The principal ligament is short, entirely internal, attached under the dorsal margin a 

 little behind the umbones, and supported or strengthened underneath by a rather strong 

 callosity, which is rounded beneath, and deeply notched at the hinder end. In addition 

 the valves are connected by a thin ligament along the hinder dorsal edge. The interior 

 of the valves is glossy, somewhat pearly, and feebly radiately grooved, the fine striae 

 corresponding with the external lirse. 



Length 23 mm., height 24, diameter 16. 



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

 Diatom ooze. 



Of this fine species only a single much crushed specimen was obtained. It differs from 

 Lyonsiella Jeffrey sii, with which it agrees in the character of the hinge and ligament, in 

 form and in wanting the fine granulation on the surface. 



Lyonsiella grandis, n. sp. (PI. XXV. figs. 3-36). 



Testa tenuis, inflata, valde injequilateralis, alba, epidermide tenuissima pallide straminea 

 plus minusve induta, liris filiformibus radiantibus crebris ornata, incremeutique lineis 

 subcancellata. Margo dorsi anticus brevissimus, posticus paulo declivis, I'ectus. Margo 

 ventral is semicircularis, postice oblique in latus arcuatim ambiens. Latus anticum 

 obliquum, in medio rectiusculum, utrinque rotundatum. Umbones magui, pileiformes, 

 jirominentes, conspicue latus versus anticum iuvoluti. Cardo edentulus. Ligament! 

 sulcus interni angustissimus, brevis. Cicatrix antei'ior formse elongatse irregularis infra 

 umbones sita, posterior major prope medium latevis. Impressiones du« minores infra 

 lineam cardinis collocatse. 



This species is very thin, considerably inflated, very inequilateral, higher than long, 

 white beneath a somewhat thin pale yellowish epidermis. The valves are very finely and 

 closely radiately Urate, and exhibit rather distinct, elevated lines of growth which almost 

 produce a cancellated appearance. 



The umbones are inflated, prominent, cap-shaped, much curved over towards the fi'ont. 



