1( ; THE VOYAGE OF II. M.S. CHALLENGER. 



( 'audal appendages with about one-fourth the length of the pedicel of the posterior 

 cirrus with long bristles at the extremity, and with the sides naked. 



Ova.— Long, oval, and pointed, their length (0'26 mm.) being exactly two and a half 



times their breadth. 



Penis clothed with very fine hairs, and bearing a tuft of somewhat stronger ones at 



the extremity. 



Size.— The capitulum of the largest specimen 14 mm. long. This specimen was 



furnished with ovigerous lamelke. 



This species was collected on two different occasions during the cruise of the 

 Challenger :— Station 24, March 25, 1873, off Culebra Island (West Indies) ; depth, 390 

 fathoms; bottom, mud. One small specimen. Station 344, April 3, 1876, off Ascension 

 Nland (Atlantic Ocean) ; depth, 420 fathoms ; hard ground. Three specimens and two 

 very small ones, attached to a Coralline. 



Pacilasma gracile, n. sp. (PI. II. figs. 2-4). 



Valves five; carina terminating downwards, neither truncated nor in an embedded disc, 

 but considerably enlarged and keel-shaped. Tergum with the basal point truncated 

 and almost parallel to the occludent margin. Tergo-lateral margin rounded. Notch 

 behind the stronger spines at the upper side of the maxillse without spines. Caudal 

 appendages with long spines at the extremity. 



General appearance. — Capitulum about twice as long as it is broad, compressed. 

 Valves white, distinctly striated. 



Scutum with the apex pointed, with a distinct ridge running to the umbo; basal 

 margin equalling the breadth of half the carina near its basal extremity. Carmo-tergal 

 margin distinctly divided into a carinal and tergal portion. The earinal portion 

 is convex and rounded, the tergal portion straight. The interior structure of the scutum 

 could not be studied, as I wished to keep the capitulum as a whole. 



Tergum basally truncated, flat, oblong, much like that of Pcecilasma kaernpferi, 

 Darwin. 



Carina very narrow and flat at its superior extremity, considerably enlarged 

 downwards and keel-shaped. The dorsal margin runs downwards over the extremity of 

 the peduncle, the base of the carina forming nearly a right angle with the basal margin 

 of the scutum. 



Peduncle about one-third of the length of the capitulum. 



Mouth. — Labrum (PI. II. fig. 3) with a row of small teeth on the crest and palpi as 

 in the other species of the genus. The mandibles (PI. II. fig. 4) have four teeth, and the 

 inferior angle terminates in two small points when seen under the microscope. Maxillce 



