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Show 1892.] CLASSIFICATION OF OPHIUROIDS. 183 (Plate XII. fig. I) shows very well the definite carination of shields ; the term carination 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 haud aequantia," hut 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 wdiich 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 decimetres. 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 specimens he obtained were in a bad condition, it is not improbable 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 XI. Fig. 1. Ophioteresis 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 (I), 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. General 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. P L A T E XII. Fig. 1. Ophiomaza obscura, upper surface of disk, to show the carinated radial shields, X 2. 2. Ophiobyrsa hystricis, from below, nat. size. 3. The same from above, nat. size. |
