1710 THE VOYAGE OF H.M.S. CHALLENGER. 



spine, placed in the prolongation of the main axis, and about half as long as the shell. Its base is 

 pierced by two large opposite pedal pores. The anterior or oral pole bears a cylindrical peristome, 

 similar to a bird's head, and curved towards the ventral face ; on both sides of its neck (at right 

 and left) a series of three or four irregular, ovate, buccal holes. The neck bears four cylindrical, 

 spinulate, radial tubes (two on each side), crossed nearly horizontally, and placed in two diagonal 

 planes perpendicular one to another ; these planes correspond to those in which the four feet of 

 Tuscarusa medusa lie. The base of each tube is pierced by four dental pores. 



Dimensions. Length of the shell 3 '2, breadth 1'6. 



Habitat. North Pacific, Station 250, depth 3050 fathoms. 



2. Tuscaridium lithornithium, n. sp. (PI. 100, figs. 8, 8a, 86). 



Shell spindle-shaped, twice as long as broad, very similar to the preceding closely allied species. 

 It differs from the latter in the following characters : The curved proboscis of the peristome is 

 broader and more spiny. The four radial tubes of the mouth and the terminal caudal spine are very 

 spiny (in the preceding species nearly smooth). The number of buccal holes (six to eight on each 

 side of the mouth) is twice as great as in the former. Each circoral tube is pierced at the base by 

 six or eight (in the former by four pores), and the base of the caudal spine exhibits a cross of four 

 pores (in Tuscaridium cygneum only two pores). 



Dimensions. Length of the shell 3 - 6, breadth l - 8. 



Habitat. North Pacific, Station 264, depth 3000 fathoms. 



Order IV. PILEOCONCHIA, Haeckel, 1879. 



Definition. PH^EODARIA with a bivalved lattice-shell, composed of two free opposite 

 valves (a dorsal and a ventral), between which the central capsule is enclosed. 



Family LXXXIII. CONCHARIDA, Haeckel, 1879 (Pis. 123-125). 

 Coneharida, Haeckel, 1879, Sitzungsb. med.-nat. Gesellsch. Jena, Dec. 12, p. 6. 



Definition. PH^EODAEIA with a bivalved lattice-shell, which is spherical or lenticular, 

 and composed of two equal or unequal boat-shaped valves, a dorsal and a ventral. The 

 valves bear neither an apical latticed cupola or galea, nor hollow radial tubes. The 

 central capsule is placed in the aboral half of the shell-cavity, and so enclosed between 

 both valves, that its three openings lie in the open frontal fissure between them (the 

 astropyle on the oral pole of the main axis, the two parapylse on both sides of its aboral 

 pole, at right and left). 



