48 THE VOYAGE OP H.M.S. CHALLENGER. 



think that it is more nearly related to Dichelaspis lowei, Darwin, than any of the other 

 species ; it corresponds to this species and to Dichelaspis darwinii, Filippi, in the form 

 of the carina. The third species is Dichelaspis aymonini, Lessona et Tapparone-Canefri, 

 from Japan. So we have eight species, which all inhabit eastern and western warmer 

 oceans in the northern hemisphere (Madeira, Mediterranean, off Borneo, China Sea, 

 Japan coast, Indian Ocean, &c). They were found attached to Sea-snakes, Crustaceans, 

 horny Corallines, &c. The individuals of most species seem to be rare, as Darwin 

 suggested. About their bathymetrical distribution almost nothing is known. 



To this genus belongs also, I believe, the species before me. It differs from the 

 typical species as described and figured by Darwin in the extent to which the valves are 

 separated from each other, and in the form of the scutum. In both regards the new 

 species is intermediate between Pa>,cilasma and Dichelaspis. However, the carina 

 extending up between the terga, and being much bowed, gives it in a striking manner the 

 appearance of the species of Dichelaspis; especially to that of Dichelaspis orthogonia, 

 Darwin, the present species shows a considerable resemblance. 



For the determination of the species known at present, I insert the following table: — 



Dichelaspis. 



1. Carina terminating in a disk. 



i. Basal segment of the scutum twice as wide as the 



occludent segment, .... Dichelaspis warwiclcii, Gray, sp. 

 ii. Basal segment narrower than the occludent 



segment, ..... Dicltelaspis grayii, Darwin, 

 iii. Basal segment much narrower and about half as 



long as the occludent segment, . . Dichelaspis pellucida, Darwin. 



2. Carina terminating in a fork. 



i. Basal segment of the scutum directed towards the 



centrum of the capitulum, . . . Dichelaspis neptuni, Macdonald, sp. 



ii. Basal segment of the scutum running parallel to 



the lower margin of the capitulum. 



(a) Capitulum almost as long as broad. 



a Tergum triangular, . . . Dichelaspis aymonini, Lessona. 



/3 Tergum divided by a deep notch, . Dichelaspis darwinii, Filippi. 



(b) Capitulum more than once and a half as 



long as it is broad, . . . Dichelaspis lowei, Darwin. 



3. Carina terminating in a cup. 



i. Scutum divided into two distinct segments, . Dichelaspis orthogonia, Darwin. 



ii. Scutum with a notch only and indistinctly divided Dichelaspis sessilis, n. sp. 



Dichelaspis sessilis, n. sp. (PI. II. figs. 10-12). 



Interspace between the scutum and the two other valves not so considerable as in the 

 other species, the scutum being only indistinctly divided in two segments. Basal segment 



