34 BULLETIN 84, UNITED STATES NATIONAL MUSEUM. 



to that of the second plate and it immediately acquires such characters as are dis- 

 played only on that second plate in the same genus. It shows on each side a large 

 pore provided with a scale, and as this pore corresponds to an ambulacral tube, 

 the result is certainly an important modification in the aquiferous system, such as 

 Mortenscn had already pointed out. It is obvious that this structure can not be 

 modified by age, and even if it were proved that the genus OpJiiomiddium includes 

 only young foi-ms, it would nevertheless constitute a generic character of the utmost 

 value. 



The second character of the genus OpMomisidium, that is to say, the widening 

 of the first lateral brachial plates, shows to a variable degree in the three species 

 of that genus which are actually kno^vn. It is especially conspicuous in . jlabellum, 

 where the first lateral plate is considerably widened and completely covers the inter- 

 radial under space Vv'hUe at the same time it leans on the median line against the 

 front one and takes the place of the corresponding genital plate; the mouth shield 

 is thus pushed back toward the mouth and remains rudimental. In 0. puIcTielhm, 

 the first lateral brachial plate, though very large, does not extend so much over the 

 interradial space, but the latter nevertheless remains very narrow and is bounded 

 chiefly on each side by the genital plate. Lastly, in 0. speciosum, which I describe 

 below, the fu-st lateral plate is less widened, and on the whole length of the inter- 

 radial space, from the end of the mouth shield to the margin of the disk, there is a 

 large median plate which separates the genital plates throughout their whole length. 

 In the last two species, the bursse find, at the lower face of the disk, a sufficient 

 space to make their opening outwardly, which docs not seem to be the case in 

 . flahellum. 



Type of the genus. — OpMomisidium speciosum, new species. 



OPHIOMISIDIUM SPECIOSUM, new species. 

 Plate 3, figs. 3-4. 



Albatross station 2415. Apr. 1, 1885. Lat. 30° 44' N.; long. 79° 26' W.; 440 

 fathoms; co. crs. s. sh. for.; temp. 45.6° F. Two specimens. 



Albatross station 2761 (type-locahty). Dec. 26, 1887. Lat. 15° 39' N.; long. 

 38° 32' 54" W.; 818 fathoms; pter. oz.; temp. 39° F. Four specimens. 



Type.— C&i. No. 32291, U.S.N.M. 



In the largest specimen from station 2761 the disk is 5 mm. in diameter, the 

 arms are also 5 mm. long; the other specimens are somewhat smaller, and the disk 

 ranges from 3.5 to 4.5 mm. in diameter. Tlie two specimens from station 2415 are 

 very small, the diameter of the disk ranging between 2 and 2.5 mm. 



The disk is rounded, thinner at the edges, and is continued, without any well- 

 defined line of demarcation, by the arms; the upper face is very strongly convex, 

 while the under face is plane. 



The arrangement of the plates of the upper face of the disk is the same as in 

 0. pulcTieUum Lyman. The central region is occupied by sbc large primary plates, 

 all being about the same size; the centro-dorsal plate is pentagonal, and the radial 

 plates have a more or less hexagonal outline. Next come, in each interradial space, 

 two successive plates; the first is very large, pentagonal, longer than wide with an 



