OPHIURANS OF UNITED STATES NATIONAL MUSEUM. 33 



admitted that peculiarity which, alone, would be sufTicient to justify a generic 

 separation. Now, I have been able to see such slits existing not only on a speci- 

 men of 0. pulcTiellum gathered by the Albatross (station 2625), but also in the new 

 species to which I allude above, 0. speciosum; these slits are exceedingly small, 

 short, and narrow, but easily discernible, and there can be no doubt as to their 

 presence. The question now is whether the absence of genital slits in O.JlaheUum 

 and their presence in the other two species prevent one from placing all three in 

 the same genus; I do not think so, as all the other characters of their structure 

 which I have observed are in conformity. 



It is beyond dispute that, owing to the considerable widening of the first 

 lateral brachial plate which, with its congener, covci-s the whole interradial under 

 space in 0. jiabcUum, there remains no vacant space for the genital slits. But 

 O.jlahellum is a very small species in which the diameter of the disk does not exceed 

 3.3 mm., and the arms also are only 3.3 mm. long; the question may be asked 

 whether Lyman's type is not a young specimen, in which case one might imagine 

 that the intercalation of new plates would, as the animal grows older, allow genital 

 slits to be formed. Whichever be the case, it seems to me that the peculiarities 

 displayed in our three Ophiurans by the ventral and lateral brachial plates are 

 amply suflicient to make the introduction of a new genus necessary, and I also 

 think that the three species which I propose to classify in this genus are too 

 closely allied for one of them to be separated from the other two, at least not until 

 the discovery of new specimens has enabled us, by an anatomical study of these 

 forms, to get some information concei'ning the state of the genital organs, chiefly 

 in O.jiabeUum. 



It will perhaps not be useless to recall, in this connection, that in 1904 I intro- 

 duced a new genus of Ophiurans which also shows a considerable development 

 of the liret lateral brachial plate, and to which I gave the name OpMomidas (04, 

 p. 26). I placed in this genus two new Ophiurans gathered by the Siboga {0. alatuin 

 and 0. reductum), as weU as a third species described in 1878 by Lyman and pro- 

 visionally classified by him, with other species, m the genus Ophiozona under the 

 name of 0. dubia (78, p. 224). It happens that in 0. alatuin and 0. dubium the 

 disk i.s very small, its diameter not exceeding .3.5 mm., and the first lateral brachial 

 plate is remarkably widened; in 0. dubium it even covere on each side half the 

 interradial under space and it jouis its congener on the interradial median line; 

 Lyman did not see in that species, nor have I seen in 0. alatum, any genital slits. 

 On the contrary, in 0. reductum, which is larger and in which the diameter of the 

 disk reaches from 6 to 7 mm., the genital sUts are visible and extend up to the 

 edge of the disk; and yet they are partly hidden by the fii-st lateral brachial plate, 

 which is much less widened than in the other two species. 



One of the two chief characters on which the genus OpTiiomisidium is based, is 

 the very peculiar stracture of the fu-st xinder brachial plate. Instead of being 

 rudimental and compressed on both sides between the adoral plates, and being 

 thus different from the succeeding plates, which are from two to four in number, 

 and are large and provided on each side with a large tentacular pore, such as is 

 most constantly observed in all the known species of the genus OpMamusium, that 

 first ventral plate here takes on a great development, for its size is somewhat superior 



