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



is marked by the greater elongation of the posterior extremity of the test, the more 

 vertically truncated anal end, the greater size of the anal system (PI. XXX." fig. 16), the 

 comjiaratively narrower and stouter lateral ambulacral petals nearly flush with the test 

 (PI. XXX.* fig. 15), the very indistinct disconnected peripetalous fasciole, the anterior 

 part of the test sloping towards the ambitus quite gradually, and the regularly elliptical 

 actinal plastron ; while in Brissus unicolor it is broadest near the subanal fasciole, the size 

 of this fasciole is also comparatively much smaller than in the undoubted Brissus unicolor. 

 The depths at which these small specimens were found seems also to indicate either a 

 well-marked variety of Brissus unicolor or more probaljly the young of a hitherto un- 

 described species of Brissus, and it will be very interesting to see what becomes of this 

 rudimentary peripetalous fasciole, or whether this is only an abnormal case of development, 

 as this type of Brissus would if adult be closely allied to Macroiyneustes with no peri- 

 petalous fasciole, and also to Micraster from the slender development of its petaloid 

 system. This species seems to hold to Brissus much the same relations which Naco2)a- 

 tagus holds to Spatangus proper. 



Dames has figured as Briss^is (comp. B. dilatatus, Des.) a small species, which seems in 

 many respects to be more closely allied to this species than any other of the genus. 

 Dames ^ gives no fascioles, and I am unable from his descriptions in the text or explana- 

 tions of the plates to determine whether the peripetalous fasciole existed or not in his 

 specimens. The general structure of the petals and of the tuberculation of that species 

 agrees well with ours, but the shape of the actinal plastron is different. 



The delicate peripetalous fasciole of a species of Pe.ripneustes'^ which Dames also 

 figures, leads me to think that Dames' Brissus may j^ossibly be the young of this genus, 

 and that the species of Brissus to which I have called attention is the living 

 representative of this Tertiary Peripneustes, which in its turn is certainly most closely 

 allied to the recent Brissus. 



We have already in Brissus damesi, where the petals are nearly flush with the test 

 and scarcely petaloid, a close approximation to such Tertiary forms as Heterohrissus of 

 ]\Ianzoni and Mazetti, in which the petaloid extremity of the ambulacra near the 

 aliactinal pole differs from the rest of the ambulacral zone in having two pairs of pores, 

 much as we find it in the embryonic petals of Spatangus and Brissopsis, in the younger 

 stages of growth ; this seems to be the very type of ambulacra which we find in such genera 

 as Pygaster and Pileus and in the Galeritid^, only limited to the abactinal region ; the 

 actinal region having already assumed the Spatangoid limitations of the simple pores 

 which extend to their junction with these x'udimentary Galeritid petals. 



' Dames, Palseontograph. xxv., III. F. i. p. 74, pi. xi. fig. 4. 



- Dames, Peripneustes does uot seem to me to Ijeloiig to Peripneustes of Cotteavi, which I take to lie a true Mcoma, 

 the species which Cotteau figures (Echinides Tert. de St Barthelemy et AngxiilLa, 1875) being closely alUed to the 

 common West India species, Meorna ventricoso. 



