1720 THE VOYAGE OF H.M.S. CHALLENGER. 



Genus 722. Conchellium,^ n. gen. 



Definition. — C o n c h a r i d a with the lateral margins of the valves dentate, without 

 sagittal keel and without horns on the hinge. 



The genus ConcheUium and the four following genera represent together the sub- 

 family Conchopsida, differing from the Conchasmida in the dentate lateral margins of 

 the two valves. These are armed with a series of strong, conical teeth, and catch one 

 into another just as the two valves of many Lamellibranchiata and Brachiopoda do 

 (PL 124, figs. 1-16). ConcheUium is the simplest form among the Conchopsida, 

 since the valves are hemispherical, and possess neither a sagittal keel nor projecting 

 horns. 



1. ConcheUium tridacna, n. sp. (PI. 123, figs. 7, 7a). 



Shell nearly spherical, finely tuberculated, the sagittal diameter somewhat longer than the two 

 others. Borders of the two hemispherical valves smooth in 0'2 of the oral, and 0"1 of the aboral 

 part, dentated in the remaining 07 part; on one side of each valve fourteen to sixteen very strong 

 and long teeth, all nearly of the same size, about one-fourth as long as the shell-radius. In the 

 half frontal perimeter of the shell (on one border of each valve) twenty-five to thirty pores, in the 

 half sagittal perimeter thirty-five to forty, in the equator twenty-five to thirty. Pores circular, 

 hexagonally framed, of equal size (except some smaller rows along the fissure), twice as broad as the 

 bars. On the conical inside of each funnel-like pore six small spinules, between every three 

 neighbouring pores a triangular facette (fig. 7a). 



Dimensimis. — Length of the shell 0-34 to 0-38, height 0-38 to 0-42, breadth 0-32 to 0-36. 



Hahitat — North Pacific, Stations 250 to 253, depth 2740 to 3125 fathoms. 



2. ConcheUium hippopus, n. sp. 



Shell nearly spherical, in the lateral perimeter (along the girdle-fissure) somewhat constricted. 

 Borders of the two hemispherical valves semicircular, smooth in 0'3 of the oral, and 02 of the 

 aboral part, dentated only in the remaining 0'5 middle part ; on one side of each valve seven to 

 eight very strong and long teeth, increasing in size towards the mouth, the longest (foremost) teeth 

 nearly one-tliird as long as the shell-radius. In the half frontal perimeter of the shell (along one 

 border of each valve) twenty-four to twenty-eight pores, in the half sagittal perimeter thirty-two to 

 thirty-six, in the half equator twelve to fourteen. Pores circular, twice to three times as broad as 

 the bars, smaller along the fissure. 



Dimensions. — Length of the shell 0-06, height 0'065, breadth 0-055. 



Hahitat. — Central area of the Tropical Pacific, Station 274, depth 2750 fathoms. 



' Co?w;^eWiMm = Small Mvalved mussel ; w/xih'huiv. 



