32 BULLETIN 84, UNITED STATES NATIONAL MUSEUM. 



Fi^h HawTc station 7281. Feb. 14, 1901. Lat. 24° 13' 45" X.; long. 81° 58' 

 15" W.; 304 fathoms; s.; temp. 52° F. One specimen. 



The diameter of the disk varies between 5.5 and 8 mm. 



In the type described by Lyman, tlie diameter of the disk was 11 mm. wide. 

 I observe between this type and the two specimens I have in hand, differences of the 

 same kind as those which I have indicated above in 0. validum, but exactly reversed. 

 The upper plates of the disk are fewer and larger, and the two radial shields of each 

 pair, instead of being parallel as represented by Lyman, are slightly diverging and 

 distaUy contiguous. The primary plates are, besides, distinct. 



OPHIOMISIDIUM, new genus. 



This genus is closely allied to the genus Ofhiomusium and up to now includes 

 only species of very small size. The plates of the upper face of the disk are few 

 and regularly arranged; the interradial spaces of the under face are extremely 

 reduced owing to the more or less considerable widening of the first lateral brachial 

 plates, chiefly of the first two; thanks to that %videning, the arms, which, by the 

 way, are very short, are broadly united to the disk and their width very rapidly 

 decreases so that they offer, on the whole, a triangular shape. The first two 

 widened lateral brachial plates carry some well-developed spines which are broad 

 and flattened. Moreover, the first under brachial plate, instead of being rudimental 

 as is normally the case in the genus OpJiiomusimn, offers exactly the same shape 

 as the two following ones, and even exceeds them in size; it displays on each 

 side a tentacular pore provided with a scale located exactly as on the succeeding 

 plates. According to the widening of the first lateral brachial plate being more or 

 less important, the interradial spaces of the under face are more or less reduced 

 and they may even be completely lacking; in the latter case the genital slits also are 

 lacking while they do exist whenever the first lateral plate is less developed. 



The genus OpMomisidium, thus defined, includes three species, two of which 

 are already known and had been classified by Lyman in the genus Ophiomusium, 

 namely 0. flahellum and 0. pulcJiellum; the tliird one, which is described below 

 under the name of 0. speciosum, is new. 



Certain peculiarities in the structure of 0. fiahellum and 0. pulchellum had 

 akeady been indicated by Lyman, when describmg these two species which had 

 been gathered by the Challenger. In 1893, in his paper on the OpTiiopus arcticus, 

 Mortensen formally stated that these two species, being deprived of genital shts, 

 had erroneously been referred to the genus OpMomusium and that they ought to 

 constitute a separate genus (93, p. 525); this naturalist backed his statement 

 also on the peculiar shape of the first under brachial plate and the widening of 

 the fu-st lateral brachial plates. I therefore am simply namijig the genus which 

 the learned Danish naturalist proved should be introduced, and yet the genus I am 

 proposing does not absolutely correspond to that which Mortensen conceived. In 

 fact, as stated above, I consider the genus OpMomisidium to be charactei-ized 

 essentially by the shape of the fir-st imder brachial plate and by the widening 

 of the first lateral brachial plates, and I introduce only incidentally in the 

 diagnosis the presence or absence of genital slits. Lyman had asserted that 

 the genital slits were lacking in 0. flabellum and 0. pulchellum, and Mortensen 



