280 BULLETIN OF THE 



simple; suture distinct, not deep. Lon. of shell, 4.0; of last whorl, 3.0; of 

 aperture, 1.8; max. lat. of shell, 2.5 mm. 



Habitat. Yucatan Strait, 670 fms., one dead specimen. 



This form is related to R. xanthias and pyrrhias of Watson, from Loth of 

 which it differs in its white and unsculptured depressed nucleus, as well as in 

 the fine and close character of its sculpture, its larger size, and relatively- 

 thinner shell. 



The shell is not umbilicated when adult, but either the young shell differs 

 in this character, or there is a variety of the species which has a well marked 

 umbilical furrow behind the outer lip, and wants the spiral grooving on the 

 base, though agreeing with the type in other respects. A not quite mature 

 shell of this description was dredged by Dr. Rush in 498 fms., off Gun Cay, in 

 the path of the Gulf Stream. 



Rissoa (xanthias Watson var.?) acuticostata Dall. 



Plate XIX. Fig. 10. 



Rissoa xanthias Watson, Cliallenger Gastr., p. 588, pi. xliv. fig. 5, 1885. 



This form has a yellow nucleus of three whorls regularly conical, and with 

 the last whorl finely spirally grooved and crossed by fine elevated incremental 

 lines. The adult has strong sculpture, fourteen very sharp suturally tuber- 

 culated ribs; the whorls spirally minutely striate, with three to five or more 

 stronger threads before the periphery, no umbilicus, and a strong varical rib. 

 Alt. 3.7; lat. 2.0 mm. 



Habitat. Ott" Cape San Antonio, in 640 fms.; Barbados, 100 fms.; Station 

 20, Gulf of Mexico, in 220 fms. Also at U. S. Fish Commission Station 2117, 

 off" Hatteras, N. C, in 683 fms., mud; and Station 2387, in 32 fms., sand. Gulf 

 of Mexico, between the Mississippi delta and Cedar Keys. 



Were I convinced that the characters of the nucleus are invariable in this 

 group, I should not hesitate to regard this as a new species. As I strongly sus- 

 pect they vary, as do the characters of the adult shell, and I have not sufficient 

 material to decide that point, I prefer for the present to regard this as a variety 

 of Mr. Watson's species, from which its apical portion differs in a marked 

 manner. 



Among the other species of this genus which have been found in deep water 

 off our coast are R. pelagica Stm. (carinata Mighels non Da Costa nee Phi- 

 lippi) ; a form not named, extremely like R. (Setici) australis AVatson ; another, 

 R. arenaria Mighels, extremely close to R. castanea MoUer; R. tenuisculpta 

 Wats.; R. deliciosa Jeffreys; R. pyrrhias and xanthias WaXson; R. Jan-may eni 

 Friele, and its smooth variety R. brychia Verrill; R. syrxjenes, leptalea, apicina, 

 Sandersoni, and harpa of Verrill ; R. areolata Stm. ; R. turgida ? Jeffreys, and 

 R. (Cingula) aculeus Gould. 



The proper classification and confirmation of these species must await a larger 

 supply of fresh material, and a thorough revision of the whole group. 



