1718 • THE VOYAGE OF H.M.S. CHALLENGER. 



marginal pores small, nearly circular; the dorsal and ventral pores oblongish-hexagonal, twice as 

 long as broad, about four to six times as long as the bars, regularly arranged in transverse rows. 



Dimensions. — Length of the shell 0'22, height 0'21, breadth 0-2. 



Habitat. — Tropical Atlantic, near Sierra Leone, Station 348, depth 2450 fathoms. 



4. Concharium hacillarium, n. sp. (PL 123, fig. 4). 



Shell walnut-shaped, with panelled surface ; oral and aboral face of the same form. Its 

 longitudinal diameter about one-fifth longer than the two other diameters. Borders of the two cup- 

 shaped valves elliptical, smooth, with a prominent edge, about as broad as the larger pores. In 

 the half frontal perimeter of the shell fifty to fifty-five pores, in the half sagittal perimeter thirty- 

 six to forty, in the half equator also thirty to forty. Pores hexagonally framed. The pores are 

 tapering in size from the sagittal plane towards the valve-margins, and' so regularly arranged in 

 meridional rows that the crests between the latter converge towards both poles of the longitudinal 

 axis. 



Dimensions. — Length of the shell 0'2, height 015, breadth 015. 



Hahitat. — Tropical Atlantic, off St. Helena, Station 340, depth 1500 fathoms. 



5. Concharium fragilissimum, n. sp. 



Shell subspherical, very thin-walled and fragile. Diameter in all directions nearly the same. 

 Oral and aboral face scarcely different. Margins of the hemispherical valves extremely thin and 

 hyaline. Pores irregularly roundish, of very different sizes and unequal forms. The fragile shell of 

 this species differs in general shape from that of all other Concharida, and is like that of the 

 Coelodendrida (PI. 121, fig. 3), but exhibits neither an apical cupola or galea, nor radial tubes 

 arising from it. It may be perhaps a young specimen of Ccelodendrum. 



Dimensions. — Diameter of the shell 0'22, of the pores 0'002 to 0-02. 



Habitat. — Mediterranean, Portofino (Haeckel), surface. 



Genus 721. Conchasma,^ n. geu. 



Definition. — C oncharida with the lateral margins of the valves smooth, without 

 sagittal keel, but with two caudal horns on the hinge (a dorsal and a ventral). 



The genus Conchasma is closely allied to the preceding Concharium, and has the 

 same hemispherical valves with smooth margins, without teeth ; but it differs from the 

 latter in the development of two caudal horns or posterior spines on the aboral hinge, 

 one horn on the aboral end of each valve. The three species of this genus were all 

 found in great depths of the Antarctic Ocean, in Diatom ooze, between 1260 and 1975 

 fathoms, at Stations 152 to 157. 



1 Oo»ici'iasma = Bivalved shell-fish; Myx'^^f^"-- 



