372 Mr. H. Seeley on Cambridge PalaontoJogy, 



have the ambulacra chiefly developed on the dorsal instead of on 

 the ventral surface. 



But though not to be included under any of these orders, 

 they are by no means unrelated to them. The concentration of 

 the viscera within the disk would indicate an affinity with the 

 Brittle-stars, while the development of the ambulacral suckers 

 would show their near relation with the true Star-fishes*. 



It has already been shov/n that the dorsal surface of the disk 

 was probably covered with large scutes ; and now the affinity of 

 the group to the Brittle-stars is evident, it cannot but be re- 

 garded as an interesting coincidence, that whereas the supposed 

 large plates are associated with large spines on the rays of these 

 animals, it also happens that the genus Ophiothrix, which has 

 large plates on the disk, is likewise furnished with large spines 

 on the rays. 



The genital openings were in all probability placed on the 

 ventral surface, much as in the Brittle-stars. Considering the 

 whole of the affinities, it would seem improbable that there 

 should have been either a vent or a madreporic tubercle ; but 

 this latter assumption is open to some slight doubt. 



From the disk not being known, the distinction of genera 

 will be difficult; and it is not impossible that generic characters 

 may be passed over as specific. But we are not entirely without 

 guidance. The width of the ambulacral groove at the point of 

 attachment varies in the diflerent forms; and this will indicate 

 a relative difference in the size of the disk — a character which, 

 from its general constancy, we shall be justified in regarding as 

 generic. 



But in order to get an idea of the relative size of the lobes to 

 the disk, it will be necessary to assume some size for the mouth. 

 The experience of its proportions in many existing forms would 

 suggest that its diameter was at least equal to, if not greater 

 than the wddth of the ray ; and as in this order the rays did not 

 touch each other, it will thus be easy, on observing the com- 

 parative width of the ambulacral groove, and the ratio in which 



* But, though to some extent inteimediate, and partially filling the gap 

 between these orders, they eannot be regarded merely as a link for blending 

 them. Its own peculiar characters require for the group an independent 

 position, which will show that it connects the allied orders rather as a 

 parallel than as a continuous chain. It will, however, introduce between 

 the Asteroidea and Ophiuroidea much such a relation as in Crustacea is 

 produced between the Macroura and Brachyura by the intervening Ano- 

 moura. Hence it will be impossible any longer to regard the Asteroidea, 

 for instance, as an order of equal value with the Echinoidea, any more than 

 the Brachyura can be compared in value with the Stomapoda. If great 

 groups are to be considered natural, it will constitute a suborder of an order 

 which will include the fossils under consideration and the Brittle-stars. 



