| OCR Text |
Show 988 MR. F. A. BATHER ON UINTACRINUS. [Dec. 17, more tl i t (he average), we conclude that the total length must be 10cm.-t t>6' cm. = 106 cm. Similarly, taking it that the height of a brachial has to decrease from 1*4 m m . to *65 m m . and that the rate of decrease is, at the utmost, *2 m m . in 20 cm., then we conclude that the total length was not less than 10 cm.+ 80 cm.= 90 cm. This latter method is not quite so reliable as the former, since measurements are more difficult to make, and since syzygies interfere wdth the striking of a correct average. But we are certainly justified in concluding that in an adult the length of a free arm-branch, counting from the suture between the eighth and ninth secundibrach, was certainly not less than 100 cm., or about 3 feet 3| inches, and that it contained fully 1000 brachials. Consequently, to adapt the words of the first describer of tbis crinoid, " it seems probable that in life the spread of the outstretched arms may have been " 6 feet 9 inches *' or more." The longest arms in other crinoids are about 26 cm., or 10i inches, in Extracrinus from Boll, and about 22 cm., or a little under 9 inches, in Scaphiocrinus swallowi from the Carboniferous of North America. Enormous though the length is, as compared with that attained by other crinoids, the draw ing of a reconstructed individual shows that it is by no means excessive when compared with the size of the dorsal cup (Plate LV.). The arm-branches are found stretched along ventralwards, or opened at right angles to the cup, or bent back aborally, sometimes straight, sometimes curved, sometimes coiled round in a loop of half an inch or less in diameter. It is clear that they had great power of motion in all directions ; and this is borne out by the structure of the brachials. The brachials are usually compressed along different axes, according as they lie on the slab, so that it is hard to estimate their exact shape. It appears that the more proximal brachials were wider than deep (PL LIV. figs. 6, 8), while the more distal ones w^ere deeper than wide (PL LIV. figs. 2, 5), also that the more distal brachials were higher in proportion than the more proximal ones. Owing to the pinnulation, the sutures between the brachials are not parallel, but slope alternately right and left. This feature, which was marked in the fixed brachials, is barely perceptible in the more proximal free brachials, but increases distal-wards. The more proximal brachials are smoothly and regularly rounded ; but the more distal ones become excavate and develop a clearly marked ridge, or cornice, on their distal margins, which ridge is more intense on the side towards the pinnule (PL LIV. fig. 11). The ventral groove is V-shaped. In the more proximal brachials it is wide and enclosed by convexly curved sides (PL LIV. fig. 6); in the more distal brachials it becomes gradually deeper and is enclosed by straight sides (PL LIV. figs. 2, 5). Covering plates to the ventral groove have not been observed ; probably they were small and lay, separate from one another, in a membrane, and so would not readily be preserved in situ. |
