70 ON THE ECHINODERMATA OF THE 



1876. AstropTiyton Agassizii, Norman, Proc. Roy. Soc. vol. xxv. p. 208. 



1877. AstropTiyton Agassizii, Lyman, Proceed. Boston Soc. Nat. Hist. vol. xix. p. 102. 

 1877. AstropTiyton arctlcum, Smith, MS. 



Disk with prominent radial ribs, which taper slightly towards their inner 

 extremities, and reach nearly to the centre, their outer extremities forming a bold 

 protuberance at the margin of the disk, above the junction of the rays. The ribs 

 bear a number of irregularly disposed, short, conical spinelets, or spiniform tubercles. 

 The disk is covered with a leathery skin ; and upon the interradial spaces, which are 

 smooth and well shield-shaped, a few granules are present. Mouth-shields either 

 quite insignificant or aborted. The side mouth-plates, which are large and irregularly 

 pyriform in outline, meet along the greater portion of their apposed sides. The jaws 

 (scutella oralia) are nearly as large as the side mouth-plates, subtriangular in form, 

 and touch along their apposed sides, except for a short distance at the aboral extremity 

 of the same. In spirit it is difficult and often impossible to distinguish the various 

 parts, in consequence of the thick investing skin. Mouth-papillae and tooth-papillse, 

 which are spiniform and slightly tapering, are undistinguishable in form ; 6-10, which 

 may, by their position, be classed as the former, are disposed on the margin of the 

 jaw-plate, whilst an irregular group stands in the place of tooth-papillse. The 

 interbrachial spaces on the actinal surface contain a few calcareous plates or 

 scales ; and at the extreme margin a closely aggregated number of these plates form 

 a band which divides the upper and lower surfaces of the disk. Under arm-plates 

 variable and divided ; side arm-plates broad and band-like. The tentacle-scales, which 

 are spiniform and not flattened, are normally four in each series, this number being 

 diminished by one or two on the portion of the ray below the first forking. Bays 

 forked dichotomously, the portions between the forks being comparatively long and 

 unequal in length. Upper surface of the rays granulated, each joint being marked 

 off by a suture-like depression, and the granules being gathered into a saddle-like 

 point on the sides of each joint. A similar sutural depression or furrow traverses the 

 dorsal median line of the ray. Towards the extremity the granules form a regular 

 double series round the joint, the space between neighbouring series being several 

 times the breadth of the band. These granules have claw-like booklets articulated 

 upon them, which are attached to a right- and left-hand knob alternately, and are 

 present in greatest numbers towards the terminal portion of the rays. The ray up 

 to the first forking has but very few granules, the stem next the disk being covered 

 with a thick investing membrane. 



Colour. According to Stimpson, the radial ribs are yellow, and the interbrachial 

 spaces brown, in the living animal. In dried examples the ribs are a yellowish brown, 

 the rays being somewhat lighter in shade, and the interbrachial spaces a rich dark 

 brown. In spirit preparations this difference is less marked, and frequently destroyed 

 altogether. 



Remarks. In the preliminary Eeport on the Echinodermata collected during 

 the Arctic Expedition of 1875-6 (Ann. & Mag. Nat. Hist. ser. 4, vol. xx. p. 468), an 



