64 THE ATLANTIC. [chap. ii. 



to give it tlie effect of being divided nearly to the centre into 

 five broad radial lobes ; these lobes are a good deal inflated, and 

 each lobe is traversed in a radial direction by two deep grooves, 

 so that a deep outer rim of the upper surface of the disk is 

 strongly fluted. The space in the centre of the disk correspond- 

 ing with the middle third of its diameter is flat, and considera- 

 bly depressed beneath the level of the outer inflated rim. The 

 whole of the surface of the disk is tessellated with a certain ap- 

 proach to regularity with strong calcareous plates, those toward 

 the periphery larger than those near the centre ; and the plates 

 bear small stump-like spines, each with a crown of spinules on 

 the free end, inserted into distinct sockets hollowed out in the 

 plates. The radial shields are long and narrow, and lie in the 

 bottom of the grooves in the radial lobes ; so that the shields of 

 each pair are separated from one another by a high calcareous 

 arch, almost a tube, formed of the inflated calcified perisom. 

 The spines are specially congregated on the central depressed 

 portion of the disk. 



The mouth-papillae are nine for each angle ; they are broad 

 and rather blunt, with the exception of the odd papillae termi- 

 nating the strong prominent jaws beneath the rows of teeth, 

 which are larger than the others and pointed. There are no 

 tooth-papillae ; the teeth are about five in a row, pointed and 

 compressed vertically. The mouth-shields are large and wide, 

 and rudely diamond-shaped ; at the outer angle the sides of the 

 plate are turned up a little, so as to form a short spout-like ex- 

 tension toward the base of the interbrachial groove. The first 

 lower arm-plates are shield-shaped, the points closing the distal 

 ends of the mouth-fissures ; and those beyond are wide and cres- 

 centic, extending across the whole width of the arm. The ten- 

 tacular scales are simple and leaf -like, one to each tentacle. 

 The side arm-plates are very large, meeting both above and be- 

 low ; they are raised distally into a high ridge marked w^ith the 

 shallow sockets of the arm-spines. The height of this ridge 



