1.92 THE VOYAGE OF H.M.S. CHALLENGER. 



is not so well marked, the lateral fascicle on the contrary is better defined, and its con- 

 nection with the so-called internal fasciole more distinct, thus giving rise to the simul- 

 taneous existence of an internal and a peripetalous fasciole. 



With the above limitations the peripetalous fasciole appears in all recent Spatangoids 

 to modify the structure of the apical portion of the lateral ambulacra. This, as has been 

 shown, is only true to a limited extent of the genera allied to Echinocwrdium. In some 

 species of Schizaster the petals lose much of the prominence they have in such species 

 as Schizaster cancdiferus and the like, while the occurrence of such a genus as Aceste, 

 in which the lateral ambulacra are not affected by the crossing of the peripetalous 

 fasciole, and in which the abactinal part of the lateral ambulacra retains simple 

 pores, as in the Pourtalesiae, goes far to show what the true nature of the internal 

 fasciole really is. 



In Aeroj^e the peripetalous fasciole descends below the ambitus, and affects the 

 structure of the apical part of the ambulacra. The ambulacral pores are all double, as is 

 the odd ambulacrum of Aceste, but show no trace whatever of a petaloid structure ; they 

 retain their Holasteroid features, if we may so call the straight rows of double pores of 

 the ambulacral zones of some Spatangoids. 



The genus Gucdteria of the Nummulite of France is the oldest of the fossil 

 genera in which we find the peripetalous fasciole extending across the petals so as to 

 become what has been called an internal fasciole. As in Aceste it does not affect sensibly 

 the structure of the petals. This fasciole holds an intermediate position between the true 

 peripetalous fasciole placed entirely outside the petals and a normal internal fasciole, 

 plainly showing that it is impossible to draw the line between these two kinds of 

 fascicles. In the Eevision of the Echini (pi. xiv. figs. 9, 11, 12), I have figured a 

 young Spatangoid referred with great doubt to Agassizia, in which this Gualterian 

 feature of the internal jjeripetalous fascicle is very marked, and in which we have a 

 lateral fasciole starting directly from an internal fasciole in an anterior interambulacral 

 area just as it would start from a true peripetalous fasciole. 



*Aero2^erostrata (Pis. XXXIIL, XXXIII.'' figs. 8-12 ; PI. XXXIX. fig. 23 ; PI. XLI. 

 figs. 7. 8). 



Aerope rostrata, Wy. Thom.son, Proc. Roy. Soc, voL xxv. p. 211; "Voyage of the Challenger, 

 Atlantic, vol. i. p. 381, fig. 99. 



Of this species specimens of two very different sizes were collected (PI. XXIII. figs. 

 1-5 and 8-12); they differed considerably, in outline especially, when seen in profile, 

 but I am not inclined to consider them as distinct species. The large specimen measm-- 

 ing 43 mm. in length was unfortunately so badly broken that it was impossible to 

 examine its structural features without danger of completely destroying it in the prepara- 

 tion of either the actinal, anal, or apical system. I have, therefore, limited myself to the 



