a new Genus of Echinodermata. 409 



connexion which is apparently traceable with the genital 

 scales at the base of the radii, it seems not improbable that 

 such may be the case. It would be hazardous, however, even 

 to surmise, with our present information, whether these ex- 

 tended interradial spaces are utilized mainly as a prolongation 

 of the peritoneal cavity, or whether, on the other hand, they 

 serve more specially the purpose of a marsupium or nida- 

 niental cavity — a function not unfrequently called into action 

 amongst Echinoderms. 



Internal Arm-skeleton. — As might be expected from the 

 abnormal character of the animal, the internal structure of the 

 radii is both remarkable and peculiar. A form of arm-skele- 

 tion or central axis is present, but of a highly modified and 

 aborted description ; indeed, from the manner in which light 

 is transmitted through the radial portion of the disk, it would 

 be at once inferred that any internal structures were of the 

 most simple and rudimentary kind, as compared with similar 

 parts in the regular Ophiuroid test. A section was made 

 through one of the radii, between the fourth and fifth under 

 arm-plates ; and although the separation of the elements was 

 not effected as satisfactorily as might be, owing to the partial 

 ankylosis of the various ossicles, which made a certain 

 amount of fracture unavoidable and entailed the consequent 

 destruction of some detail, the section is sufficient to show 

 the general features of this portion of the anatomy of Astro- 

 pliiura without risking further damage to the specimen. 

 The body or axis of the arm-skeleton is small and slender, 

 and situated very high in position in the dorsal portion of the 

 ray (figs. 7, 8, d, e) • in fact the inferior longitudinal notch 

 is situated entirely below, instead of being (in part at least) 

 excavated out of the lower portion of the axis ; the alas or 

 wing-like disk-processes, which in the Ophiuroid arm occupy 

 the whole space between the side arm-plates, are here quite 

 small and rudimentary, being reduced to the diminutive ear- 

 shaped structures marked b. The inferior longitudinal 

 neuro- vascular notch is very large and triangular in section, 

 being, in fact, most extraordinary in size, as well as remarkable 

 for its great extension upward. 



We now come to a very noteworthy feature in the internal 

 anatomy of the present animal, which is presented by the 

 septa dividing the tentacular compartments. These consist 

 of large, broad, thin plates Avhich join up to the aborted disk- 

 processes of the axis (figs. 7, 8, a) , and form partitions reaching 

 up to the inner surface of the abactinal wall of the test, their 

 actinal edge forming the prominent straight divisions which 

 are seen, on the superficial aspect of the underside of the 



