82 PEOr. p. M. DUNCAN S EEVISION OP THE 



they have smooth tubercles as a rule. It is the single primary 

 plates of the ambulacra without any compound ones, coupled with 

 the small tubercles of the ambulacra and the few large primaries 

 near the ambitus, all the rest of the test being granular, that sepa- 

 rate the type from Sypodiadema and Hemicidaris and Diadema. 

 It is very interesting to find such a simple form so low down in 

 the Mesozoic series, and it may well have been the precursor of 

 the genera just mentioned. The similarity of the genus to Asro- 

 mlenia is striking ; but there is evidence that the sur-anal plates 

 were not present. 



If more was known about Sypodiadema, the alhance of the 

 new genus with the oldest Mesozoic forms would be very defi- 

 nite ; but the good drawings of Desor and other describers show 

 that the interradia of the genera difter considerably. 



MM. Peron and Gauthier have described GypJiosoma Heiiizi, 

 and notice its abnormal nature ; and they consider that had it 

 been placed in Pseudodiadema it would have been equally erratic. 

 Probably the classificatory position is near Orthopsis, for the pairs 

 of pores are in simple, low, primary plates. I have diagnosed a new 

 genus for the form, and dedicate it to M. Peron. 



Grenus Peeonia, gen. nov. 



Syn. Cypliosoma, Cotteau, Peron et Grauthier, 1884, Ech. 

 Foss. de I'Algerie (2nd edit, of fasc. 2), jd. 96, pi. ix. 



Test small, circular in tumid ambital outline, very depressed. 



Apical system annular, pentagonal, symmetrical ; periproct 

 large, circular ; basal plates equal, broad, low, perforated largely ; 

 radial plates small, all entering the ring and reaching the peri- 

 proct, the margin of which is slightly raised. 



Ambulacra narrow, with narrow poriferous zones, the pairs 

 not numerous, and in simple vertical series throughout, each in a 

 low primary plate. Interporiferous areas with small granules only. 



Interradia broad, with only seven coronal plates on a side, with 

 as many crenulate, imperforate and scrobiculate tubercles, largest 

 at the ambitus, the rest of the plate granular ; secondary tubercles 

 absent. 



Peristome large, decagonal, the branchial incisions with everted 

 edges. 



Fossil. Cretaceous (JN^eocomian) : Algeria. 



