70 Dr. T. Wright on Fossil Echinoderms 



British Museum, Bristol Museum, and the Collection of the 

 Earl Ducie. The specimen in our cabinet is from Santa Manza, 

 and was sent us by M. Michelin. 



Clypeaster folium, Agassiz. 

 i 



SYN. Clypeaster folium, Agassiz andDesor's Cat. rais., Ann. Sc. Nat. 

 torn. vii. p. 131. 



Test subheptagonal, much depressed; borders thin and sharp 

 like Scutella; the petaloid ambulacra short, open, and ex- 

 panded below ; acutely lanceolate at the apex ; ambulacral 

 rosette small, and rising gently from the middle of the dor- 

 sum ; poriferous zones lie in angular depressions ; apical disc 

 small, with a central prominent madreporiform tubercle. 



Dimensions. Antero-posterior diameter 11 J inch, transverse 

 diameter l^f inch, height r 3 <jths of an inch. 



Description. The general outline of this little Urchin, with 

 the structure of its ambulacral rosette, clearly prove it to be a 

 Clypeaster, whilst its depressed dorsal surface and thin border 

 show it to have affinities with Scutella. Its outline is subhep- 

 tagonal, with the anterior border slightly produced ; the petaloid 

 ambulacral areas are short and widely expanded below, tapering 

 and acutely lanceolate above; their apices closely approach at 

 the vertex, and meet at the circumference of the madreporiform 

 tubercle ; the poriferous zones lie in angular depressions of the 

 test, which, added to the convexity of the ambulacra, give a 

 much greater relief to the petaloidal star than in other congeneric 

 forms ; the rosette formed by the petaloid portions of the ambu- 

 lacra is small, being only a little more than one-half the diameter 

 of the antero-posterior axis. The two rows of pores in the pori- 

 ferous zones diverge gently from each other from the apex to the 

 base, and there are from thirty to thirty- six pairs of holes in 

 each zone. At the junction of the test-plates there are slight 

 depressions on the surface, corresponding to the sutures between 

 the same ; the tubercles are small and set rather closely together, 

 and the intervening granulation is quite microscopic ; the border 

 is exceedingly thin and entire ; the base is concealed by firmly 

 adherent matrix, which cannot be removed without fracturing 

 the test. 



Affinities and differences. C. folium is allied to the young 

 condition of C. marginatus, but the general flatness of the dorsal 

 surface, and the absence of the campanulate elevation of the 

 ambulacra in that species, added to the greater wideness of the 

 basal opening of the petaloid ambulacral areas, and the more 

 angular depression in the poriferous zones, afford points of 

 comparison whereby these two species may be distinguished 



