1722 THE VOYAGE OF H.M.S. CHALLENGER. 



3. Conchidium rhynchonella, n. sp. (PI. 124, fig. 3). 



Shell with three different dimensive axes in the proportion = 6:5:4. Ventral valve semi-ovate, 

 larger than the humpbacked dorsal valve. Margins of the valves dentate almost in the whole 

 periphery ; on each side of one valve twelve or thirteen strong conical teeth. Aboral hinge with 

 two 'very unequal horns, the dorsal much smaller than the pyramidal ventral. Under lip of the 

 mouth emarginate, much larger than the acute upper lip. Pores subregular, circular, twice as 

 broad as the bars. 



Dimensions. Length of the shell 0'3, height 0'25, breadth 0'2. 



Habitat. North Pacific, Stations 244 to 253, surface, and at various depths. 



4. Conchidium dimerella, n. sp. 



Shell laterally compressed, very similar to the preceding, but differing in the following 

 characters : Proportion of the three axes = 7:5:4. Ventral valve hemispherical, larger than the 

 humpbacked dorsal. Margins of the valves dentate in the middle half only, whilst the anterior 

 and posterior quarters are smooth ; on each side of one valve six or seven strong conical teeth. 

 Ventral horn of the aboral hinge very large, pyramidal, one-third as long as the shell and three 

 times as long as the dorsal. 



Dimensions. Length of the shell 0'35, height 0'25, breadth 0'2. 



Habitat. North Pacific, Station 236, surface. 



5. Conchidium leptcena, n. sp. (PI. 124, figs. 4, 5). 



Shell laterally compressed, with three different dimensive axes of the proportion = 4:3:2. 

 Ventral valve larger than the dorsal, both semi-ovate. Margins of the valves dentate along the 

 lateral sides ; on each side of one valve nine or ten conical teeth, the middle of which are smaller. 

 Aboral hinge with two unequal short horns ; the dorsal rudimentary. Under lip of the mouth 

 pointed, much larger than the truncate upper lip. Pores circular, about twice as broad as the bars. 



Dimensions. Length of the shell 0'2, height 015, breadth 01. 



Habitat. Central Pacific, Stations 271 and 272, surface, and at various depths. 



6. Conchidium argiope, n. sp. (PI. 124, figs. 7-9). 



Shell laterally compressed, with three different dimensive axes of the proportion = 6 : 5 : 3. 

 Both valves nearly equal. Margins of the shell dentate along the lateral fissure, on each side of 

 one valve thirteen or fourteen thin conical teeth. Aboral hinge with two equal, short, pyramidal 

 horns. Mouth with two equal short lips. Pores subregular, circular, twice as broad as the bars. 



Dimensions. Length of the shell 0'2, height 017, breadth 01. 



Habitat. Tropical Atlantic, Stations 341 to 349, surface, and at various depths. 



