288 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.48. 



Tentacles short, 12 in number, with about 8 pairs of minute digits 

 around the external, flattened surface; tips of digits tinged with 

 brown. Deposits of general perisome sparsely scattered, except on 

 the ventral side; they are wheels much resembling those of M. rinkii 

 except in their larger size, the greater number of the inward proc- 

 esses and in the latter being slightly shorter. Diameter of wheels 

 215-365/*, with a mean of 282/z. Spokes 12-21 in number, averaging 

 17; inward processes 22-33, averaging 27. In tentacles are found 

 very characteristic deposits which remind one of the Acanthotroclius 

 wheel (pi. 11, fig. 36). They vary very much in size as well as in 

 shape. Nave is solid, very often with a minute knob or so-called 

 " stalk," or sometimes a stellate figure, at the center; spokes not 

 bent and all in one plane; felly with short teeth on its internal and 

 external sides, usually more numerous than the spokes; both the 

 internal and external teeth are often inconspicuous or may be totally 

 absent. Diameter of these wheels, including the outward processes, 

 55-205/*; number of spokes, 8-21; that of the outward processes, 

 19-27. Calcareous ring very stout, consisting of 10 segments, which 

 measure 1.5 mm. in breadth, and with one or two spines anteriorly. 

 Polian vesicle and stone-canal single, the latter with calcareous net- 

 work in its wall. Genital tubes divided, in two tufts, immediately 

 behind the calcareous ring. No ciliated funnels. 



This species differs from all others of the genus in having the char- 

 acteristic spinous wheels in the tentacles. Acanthotrochus mirdhilis 

 Danielssen and Koren differs from this species in having in the 

 general perisome large spinous wheels with 8 spokes and wing-like 

 expansions to each spoke. The new species is named in honor of 

 the late Prof. Kakichi Mitsukuri, to whom the advancement of our 

 knowledge of Japanese holothurians is in a large measure due. 



Habitat. — Off the coast of Shiribeshi, Hokkaido. 



Type.— Cat. No. 34188, U.S.N.M. 



