68 RHINOBRISSUS MICRASTEROIDES. 



Spatangoids in the geological series. The thin, narrow, ill-defined peripeta- 

 lous fasciole crosses the petals without affecting their structure, as it does in 

 all the recent Spatangoids. The ambulacra are not petaloid, the ambulacra! 

 plates arc large, the pairs of pores are distant, and extend nearly to the 

 ambitus. Homolampas fulva A. Ag., among the Challenger Echini, has 

 similar embryonic lateral ambulacra, with large plates, and Hush with the 

 test, Gualteria alone among the fossils representing a similar stage of this 

 structure in Rhinobrissus. 



The indefinite peripetalous fasciole existing with a well defined broad 

 and prominent subanal fasciole indicates that in Spatangoids the fascioles 

 either may have become developed from the peripetalous fasciole. first 

 making its appearance in such genera as Hemiaster, in which, however, the 

 subanal. lateral, and anal fascioles are wanting ; or may have developed 

 mainly from the subanal fasciole in such genera as Micraster ; Rhinobris- 

 sus in the stage here figured (PI. XXIII. Figs. 3, 4) representing a Micraster 

 stage to which has been added an indistinct peripetalous fasciole, while 

 Periaster would represent the Hemiaster stage with rudimentary subanal 

 and anal fascioles. Another species of the genus collected by the Challenger 

 represents a later stage of development, with sunken lateral ambulacra, a 

 peripetalous, an anal, and a subanal fasciole indicating affinities with the 

 more specialized recent Schizasteridse. 



In a fragment of the upper part of the test, which undoubtedly belonged 

 to a specimen of this species measuring at least 30 mm. in length, the lat- 

 eral ambulacra were as yet scarcely sunken, and almost (lush with the test, 

 the anterior ambulacrum, however, towards the ambitus, being indented 

 and sunken, much as in the genus Homolampas. This fragment is interest- 

 ing, as it shows that the peripetalous fasciole is not continuous, disappearing 

 in the anterior interambulacral areas before it reaches the odd ambulacrum. 

 The subanal fasciole, judging from a fragment of that part of the test, must 

 have been remarkably prominent. 



It is possible that a young Spatangoid which I figured in the Revision of 

 the Echini (PI. XIV. Fig. 11) may turn out to be the young of this species 

 of Rhinobrissus. It has, like this species, a peripetalous fasciole crossing the 

 petals \\ ithout modifying their structure ; and large ambulacral plates. It 

 differs from it. however, in having a continuous lateral fasciole passing under 

 the :u\a\ system, as in Agassizia. 



'I'liis species of Rhinobrissus (A', micrasteroides) will probably form the basis 



