24 ASTROTOMA AGASSIZII. 



regularly imbricated scales. Under the skin may be found the radial shields, 

 which are perhaps smaller than in any other species among Ophiuridce. 

 There are no upper arm-plates, nor do the side arm-plates meet above ; 

 a want which is not found even in Ophinmi/xa, whose upper arm-plates are 

 divided in several pieces, after the analogy of Ophioplocus, and are covered 

 bv a thick skin. 



ASTROTOMA* gen. nov. 



Disk and arms granulated. Radial ribs low and narrow, running to centre 

 of disk. Arms simple, and traversed by annular ridges bearing microscopic 

 spines. Tentacle-spines stout, erect, standing by all the pores except those 

 close to the mouth. No mouth-papillas. Teeth and tooth-papillfe similar and 

 spiniform, arranged in a clump at the inner point of the mouth-angle. Two 

 genital openings lying at the outer corners of each intei'brachial space. 



This simple-armed Astrophyton resembles Astronyx in its dentition, while 

 in the covering of the disk and arms and in the tentacle-s^Dines it is between 

 Astroschema {PI. IV. f. 57, 58) and Astrogomplms. 



Astrotoma Agassizii sp. nov. 

 PI. IV. f. 62-56. 



Special Marks. — Radial ribs slightly marked. Disk finely granulated ; the 

 under side of arms and surface round mouth more coarsely granulated. 

 Tentacle-spines three or four; short, thick, slightly flattened, standing erect. 

 Upper surface of arm with low annular ridges which bear microscopic spines. 

 One madreporic shield. 



Description of an Individual. — Diameter of disk, 20°"°'. Width of arm with- 

 out spines, 4°"". Length of arm about ISS"""'. Tooth-papillaj and teeth 

 represented by a clump of more than a dozen short, stout, sharp spines, like 

 tliose of Astrophyton, and arranged at the inner apex of the mouth-angle, 

 which has no spines near its outer corners. Arms cleanly rounded above ; 

 flattened below, where they are covered by a smooth gi'anulation extending 

 to the mouth region, which is separated from the interbrachial spaces by 

 lines of coarser grains connecting the bases of the arms at the point where 

 they blend in the disk. The arched upper surface of the arm is divided into 



* afjTTjp, star ; TOfi^, cut. 



