1892.] CLASSIFICATION OF OPHIUROIDS. 183 



(Plate XII. fig. 1) shows very well the definite carination of the radial 

 shields ; the term cariualion has been and is applied to keels varying 

 so much in depth that it is difficult to gauge how slight or how great 

 it may be in any particular case. Herr Ljungman says "Brachia 

 longitudine diametrum disci ter hand sequantia," but in the specimen 

 before me the proportions are nearly 5 to 1 ; I cannot suppose that 

 this difference is of specific value. 



7. Ophiobyrsa hystricis. 



The largest Streptophiurid found within the British area is the 

 species so named by Mr. Lyman. Readers of Sir Wyville Thomson's 

 ' Depths of the Sea' will remember that (on p. 123) there is a brief 

 account of " a very large Ophiurid with thick arms, upwards of 

 three decimetres long, and a large soft disk resembling that of 

 Ophiomyxa, to which genus it seems to be allied. The specimens 

 which have been hitherto procured are scarcely sufficiently perfect 

 to allow of its being thoroughly worked out." There is not complete 

 concordance between these measurements and that of Mr. Lyman, 

 who gives the length of the arm as 187 mm. ; as the diameter of the 

 disc is 20 mm.-, the total spread would be very nearly four deci- 

 metres. 



Among the specimens collected by the Rev. W. Spotswood 

 Green during the dredging-expedition of the ' Flying Fox ' off the 

 S.W. coast of Ireland was one example of this species ; it is a good 

 deal broken and was, most unfortunately, dried. The figure, 

 however, now given of it (Plate XII. fig. 2) will give a good idea of its 

 general appearance. The diameter of the disk is 35 mm., and the 

 arms must have been at least 310 mm. long. As Thomson states that 

 the specim.ens he obtained were in a bad condition, it is not improb- 

 able that he did not preserve his largest but merely noticed its size. 



Mr. Green dredged this example in 315 fathoms ; the ' Porcupine' 

 found specimens in 345 fathoms. 



EXPLANATION OF THE PLATES. 

 Plats XL 



Fig. 1. Oj)Moteresis elegans, nat. size. 



2. Disk and arms from above, to show the large radial shields, X 8. 



3. The same from below, X 8. 



4. Aboral surface of arm-ossicle, to show the double dorsal plates, the 



simplicity of the articular cavities (a), the absence of a covering plate 

 to the ventral surface of the ossicle, the form and position of the side 

 arm-plates (?), and the position of the spines. X 24. 



5. Adoral surface of arm-ossicle, to show the double dorsal plates {d) and 



the simple articulating convexities. 



6. G-eneral view of a young Pectinura, to show the preponderating size of 



the calycinal plates, X 2. 



7. Disk of the same X 8, to show the form and character of the calcyinal 



plates. 



Plate XII. 



Fig. L Ophiomaza obscura, upper surface of disk, to show the carinated radial 

 shields, X 2. 



2. Ofhiohyrsa hystricis, from below, nat. size, 



3. The same from above, nat. size. 



