1895.] 



ME. P. A. BATHER ON UINTACHHSTS. 



987 



to touch thau 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 chan- 

 nelled " ; 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 Marxnpites or in some Fkili/crini, were unnecessary. 



Tlie arm-branches are the free continuations of the fixed secundi- 

 brachs or distichal series, and are therefore 10 in number. They 

 are composed of brachials and pinnules. 



As to the length of the arm-branches, Grinnell (2) said " it 

 seems probable that in life the spread of the outstretched arms 

 may have been two feet or more." In the w-ords of Meek (3), 

 " Mr. G-rinnell 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 : — 



Descriptioii of portion measured. 



Length of 

 fragment. 



Width of 



Brachials 

 in luui. 



Height of Bra- 

 chials in mm. 



Large specimen, beginning at IIBr, 



Smaller individual, ditto 



Isolated fragment 



J) )' 



»» »' 



)» »> 



Finials 



iy-0 cm. 

 23-5 cm. 

 22 cm. 

 18-0 cm. 

 23-5 cm. 

 13-8 cm. 

 3'5 cm. 



7 to 40 

 (V7.5 to 2-5 



3-2 to 2-.') 



2-5 to 2-0 



2-6 to 20 



2 to 1-5 



1-0 to -8 



1-4, 1-4.5, 1-2 

 1-36, 1-2, -9.5 

 1-4, 1-2 



10, 



1-25, 



•()5 



•95 

 1-1 

 •85 



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 pi'oximal portion of the 

 arm-branch, but becomes much less distalwards. Similarly the de- 

 crease 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 eacli arm-branch for the proximal irregularities ; 

 then, taking it that the width has to decrease from 4 mm. to 8 mm., 

 and that the rate of decrease is '5 mm. in 15 cm. (which is rather, 



