| OCR Text |
Show 1895.] MR. F. A. BATHER ON UINTACRINUS. 987 to touch than to sight. Each plate is slightly convex, and shows no traces of ornament or of radiate structure. Clark (8) describes the sutures between the cup-plates as " generally slightly channelled " ; there are in the British Museum specimens a few obscure indications of irregular vertical groovings in the sides of the plates, which are no doubt the channellings to which Clark alludes. The number and thinness of the plates in the cup would be enough to give it the flexibility which it clearly possessed: hence elaborate sutures, as in Marsvpites or in some Platycrini, were unnecessary. The arm-branches are the free continuations of the fixed secundibrachs or distichal series, and are therefore 10 in number. They are composed of brachials and pinnules. As to the length of the arm-branches, Grinned (2) said " it seems probable that in life the spread of the outstretched arms may have been two feet or more." In the words of Meek (3), " Mr. Grinnell thinks they may have attained a length of not less than one foot." Both Clark (8) and Hill (9) quote Grinnell as saying that the arms reached 2 feet in length, which he did not say. The longest arm-fragment yet measured is that which Hill " traced for seventeen inches," but it is not stated whether this was traced up to the dorsal cup or no. The following measurements are from specimens in the British Museum :- Description of portion measured. Large specimen, beginning at IIBrH Smaller individual, ditto Isolated fragment » » )> » » >> Finials Making allowances for some of these examples belonging to youthful individuals, we note that the rate of tapering is very gradual, that it is rapid in the immediately proximal portion of the arm-branch, but becomes much less distalwards. Similarly the decrease in height of the brachials is very gradual; it actually seems to increase at first in some cases, and then to lessen slowly. Let us allow 10 cm. in each arm-branch for the proximal irregularities ; then, taking it that the width has to decrease from 4 m m . to 8 mm., and that the rate of decrease is *5 m m . in 15 cm. (which is rather Length of fragment. 19-0 cm. 235 cm. 220 cm. 180 cm. 23*5 cm. 13-8 cm. 3"5 cm. Width of Brachials in mm. 70 to40 6-75 to 2-5 3-2 to25 25 to 2 0 2*5 to 2-0 2 0 to 1-5 1-0 to -8 Height of Brachials in m m . 1*4, 145, 1*2 1-36, 1*2, -95 1*4, 12 10, -95 1-25, 11 •9, -85 •65 |
