THE OPIIIURIDiE. 



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Other full grown specimens differ very little from that described. The 

 granulation is not always so richly developed, especially the side arm-plates 

 may be perfectly smooth. The shape of the mouth shield and the adjoining 

 plate is rather variable. Sometimes there is only a single row of scales in 

 the interbrachial spaces on the dorsal side. 



In very young specimens (diameter of disk 4-5 mm.) the granulation is 

 not yet developed; the first ventral plate is pentagonal with an angle in- 

 wards, and all the ventral plates are widely separated ; only 3-4 arm-spines. 

 The mouth shield is pentagonal, not longer than broad ; the large plate out- 

 side the mouth shield occupies, together with the large genital scales, almost 

 the whole under side of the disk, only a pair of small plates being seen at 

 the margin. On the dorsal side of the disk the six primary plates are very 

 conspicuous, thick and high, as large as the radial shields, which are in 

 mutual contact only at the outer end, separated within by a small scale; 

 among the primary plates are also seen some small scales. ■ The inter- 

 brachial space is covered by a large marginal plate and a smaller one inside 

 that. Only one of the rudimentary dorsal plates yet developed. (Lymais', 



76, Figs. 55-57.) 



Numerous specimens from Stations 3362, 1175 1.; 3363, 978 f. ; 3364, 

 902 f; 3371, 770 f.; 3376, 1132 f . ; 3392, 1270 f.; 3393, 1020 f . ; 3407, 

 885 £ ; 3411, 1189 f. ; 3431, 995 f . , 3432, 1421 f. 



Lyman (Challenger) records this species from the Pacific (88° 2' W., 

 S"" 6' S.), but he has only seen an arm of it. Evidently it is very common 

 in this ocean, as well as in the Atlantic. 



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The 24 species of Oj)Juomiismm now known are easily subdivided into 

 two groups : those which have ventral plates in most of the length of the 

 arm, and those which have ventral plates only on the 3-5 first joints. 

 The former group, again, seems to need to be divided into those in which 

 the genital slits do not reach the margin of the disk, and those with very 

 large genital slits, reaching the margin of the disk. To the latter group 

 belong 0. famillare Koeliler, 0. clegans Koehler, our 0. Diomedcm and 0. 

 variaUle, and (perhaps) 0. laqucatum Lym. The two species 0. fiahdlum 

 Lym. and . piikhelliim ^N y v . Thoms. ought not to be reckoned as OpUo- 

 musium, as they differ from all the other species in having pores at the sides 

 of the first ventral plate. As said above under Opliioglyplia, it must be 

 maintained as one of the most important characters of the genus Opldo' 

 nmsium that there are no tentacle pores visible to the first ventral plate. 



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