anew Genus of Echinodermata. 411 



Second side arm-plate, length to breadth 0-5 : 2-75 



Mouth-shield (madreporite), length to breadth 09 : - 8 



Side mouth-shields, „ ,, 1*25 : 0'75 



First under arm-plate, „ „ 1*26:1*4 



Second under arm-plate, „ „ O'O : 0o5 



Outer margin of a mouth-piece to inner point of milliin. 



opposite first under arm-plate 2*2-*> 



Length of spines in the marginal fringe : — 



a. At the extremity of a side 0*3 



/3. At the middle of a side 0*75 



Length of three segments of rudimentary arm . . 1*3 



Breadth of rudimentary arm at base 0-85 



General Conclusions. — The following peculiarities in the 

 structure of Astrophiura suggest themselves as worthy of 

 special attention, in consequence of their important bearing 

 upon the question of the affinities and relationship of the 

 form. 



1. The pentagonal, flattened, goniodiscoid body, combining 

 within its area representatives of the whole free radial system 

 as well as of the disk of a typical Ophiuran, presents us with 

 nothing less than the anomaly of an organism having Ophiu- 

 roid ray-plates expanded and then consolidated along with 

 their disk into a pentagonal Asteroid form of test. Such a 

 remarkable arrangement of the entire brachial series and ab- 

 normal development of the side arm-plates is, so far as I am 

 aware, quite without parallel amongst the Ophiuroidea *, and 

 constitutes a character which I regard as an approach to the 

 structure of the Asteroidea. Although it may be asserted 

 that the resemblance is largely superficial, and that the plates 

 when taken individually are truly Ophiuroid after all, the ob- 

 jection is counterbalanced by the fact that this combination of 

 radial and interradial systems within a common periphery 

 involves morphologically a principle of much higher import 

 than simply outward form, and to which testimony is borne, 

 more or less fully, by each of the following particulars. 



* Mr. Theodore Lyman, writing some months ago, has kindly drawn 

 my attention to an interesting species of Ophiomusium which he described 

 amongst the material obtained by the * Challenger ' expedition, O. Jla- 

 bellum, Lyman, in which a monstrosity occurs in the side arm-plates, the 

 first pair meeting outside the mouth-shield in the median line of the 

 interbrachial space. If I am not mistaken, the same peculiarity is found 

 also in Ophiozona (?) dubia, Lyman (" Blake" Exped., Bull. Mus. Comp. 

 Zool. vol. v. p. 224, pi. ii. figs. 19-21). It is difficult, however, to look 

 upon these cases as in any way fairly homologous with the extraordi- 

 nary development present in Astrophiura, in which we have the 

 series of side arm-plates produced as flat plates extending from the 

 upper arm-plates and occupying entirely a dorsal position upon the 

 test, whilst in the instances cited above the prolongation of the plates 

 occurs on the actinal surface and in a single segment of the rav onlv. 

 Nevertheless it is a very interesting feature. 



