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124 



THE OnilURIDiE. 



does not open into the mouth space. Tentacle scales; first pair of pores 

 with 6-;7 on inner, 8-9 on outer side; second with 5-6 on inner, 6-7 on 

 outer side; and thus the number gradually diminishes 5 from the eighth pore 

 none on the inner side^ from about the twelfth only one on the outer side. 

 At the distal end of the tentacle pore there is a small but distinct supple- 

 mentary plate, which connects the outer corner of the ventral plate with 

 the oviter edge of the side arm-plate, as in 0. tumulosa. At the first pore 

 this plate is also to be seen, but here it lies proximally to the first ventral 

 plate. 



Color in alcohol : gray. 



The numerous specimens differ very little from the one described. The 

 number of tentacle scales is somewhat variable. One specimen had, at the 

 first pore 3-4 on inner, 5-6 on outer side ; at the second, only 2 on inner, 

 4 on outer side ; from the seventh there was only one on the outer side. 

 The mouth shield is sometimes almost without restriction on the lateral 

 sides, and it then is almost the same as that in 0. tumulosa. As a whole, 

 these two species are very nearly i-elated, but there is no great difficulty in 

 distinguishing them : the incisure in the disk (including 2-3 rudimentary 

 dorsal plates in 0. tumulosa^ almost wanting in 0, planar specimens of the 

 same size being compared), the arm-comb (almost invisible from above in 

 0. tumulosa, well developed, often continuous over the base of the arm in 

 O. plana) y the dorsal plates (in mutual contact until about the 20th joint 

 in 0. tumulosa^ only to the 7th-9th joint in 0. pland)^ giving sufficient char- 

 acters. Of the specimens of 0, plana none exceed the size of 15 mm. in 

 diameter of disk, whereas 0. tumulosa grows much larger. 



In very small specimens (diameter of disk 4-5 mm.) the ventral and 

 dorsal plates are separated from the second or third. A few small speci- 

 mens had the inner angle of the mouth shield more acute than common. 

 The upper arm-spine not always distinctly longer than the others in these 



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small specimens. 



Stations 3357, 782 f . ; 3360, 1672 f . ; 3361, 1471 f . j 3374, 1823 i.; 

 3376, 1132 f. ; 3381, 1772 f. ; 3392, 1270 f. ; 3398, 1573 f. ; 3399, 1740 f. ; 

 3400, 1322 f. ; 3413, 1360 f. ; 3414, 2232 f. 



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