2o6 DISCOVERY REPORTS 



of second pair) united interradially ; directly above them is the united pair of first infero- 

 marginals, conspicuously larger than in Odi?ielh, their inner ends broadly engaging the 

 lower outer end of interradial plate, which is much more conspicuous superficially than 

 in Odinella. 



Structure of actinostomial RiNG(Fig. H). The relative size and position of the plates 

 of the ventral skeleton of the disk, known as the actinostomial ring, is usually super- 

 specific. To a certain extent their form is also characteristic of genera rather than species, 

 especially in the case of the mouth-plates. 



The actinostomial ring consists of two concentric circles of plates intimately joined. 

 Each radial sector of the inner circle consists of the first pair of ambulacral plates locked 

 between a single mouth-plate on either side (Fig. H, 6, 7). The odontophore, called the 

 interradial plate, forms a keystone above the mouth-plates, which are further bound 

 together by muscles (Fig. H, 6, 0', 0"). The outer circle consists, radially, of the second pair 

 of ambulacrals, and a pair of first adambulacrals ; interradially, of a pair of inferomargi- 

 nals whose size and position is of considerable importance (Fig. H, i, 6, 8, m). Also of 

 importance is whether the joint between the first and second adambulacrals is a muscular 

 or a non-muscular symphysis (syzygy). 



The structure of the actinostomial ring may not be ignored if the affiliations of a 

 species, new or old, are to be determined. The Brisinga of Asbjornsen, G. O. Sars, 

 Sladen, and Ludwig was not homogeneous. The relations of the plates of the ring gave 

 the clue to a subdivision into more natural generic groups. The same is true of Freyella} 



In Fig. H is recorded the structure of the actinostomial plates of Odinella and for 

 comparison, those of Brisingello. This comparison must be worked out in connection 

 with the explanation of the figures, as a description would be of doubtful value. Atten- 

 tion may be called to certain significant points. In the interbrachial angle, the relation 

 of the first adambulacrals and first inferomarginals, inter se, presents almost the extremes 

 of contrast in Odinella and Brisingella. Of a type similar to that of Odinella is the 

 arrangement in Odinia, Brisingenes, Brisingaster, Brisinga, Craterobrisinga, Astroste- 

 phane. Irrespective of details of difference in the ring plates these genera can be separated 

 as follows: 



a^. Papulae on disk and rays; no true syzygy between first and second adambulacrals; gonads 

 2 to each ray. 

 b^. Genital area elongate with stout intercostal plates on side of rays; no brood pouch. 



Odinia. 

 Z>2. Genital inflation very short, without intercostal plates on sides of rays; specialized 



brood pouch. Odinella. 



a''. Papulae on disk only; a syzygy between first and second adambulacral plates; gonads in 



series on each side of ray. Brisingenes. 



d^. No papulae ; a syzygy between first and second adambulacral plates. 

 h^. Gonads 2 to each ray. 



c'. Integument between costae without plates. Astrostephane. 



c~. Integument between plates crowded with spineless plates. Stegnobrisinga. 



1 Fisher, Ann. Mag. Nat. Hist., ser 8, xx, 1917, p. 418; also, 1919, p. 502; 1928, p. 4. 



