I 

 254 CARATOMUS 



poriferous zones together Avith a portion of inter-ambulacra is shown, consisting of 

 three plates magnified six diameters ; the arrangement of the tubercles is most accurately 

 given, and their relative size and structure well shown in this drawing. 



Afinities and Differences. — This species resembles very much Trematopygus C'amjrl- 

 cheanus, d'Orbigny. Our Urchin is larger and more gibbous, and the anal sulcus wider 

 and more developed ; the base likewise is more undulated, from the inflation of the basal 

 portions of the inter-ambulacra ; and the arrangement of rows of granules above the pores 

 in the poriferous zones, ' Pal. Fran^aise,' pi. 950, fig. 0, is absent in T. Farinc/donensis. 

 In the absence of specimens with which to compare these nearly allied forms, it is 

 impossible to decide whether they are specifically distinct or only varieties of T. Olfersii. 



Locality and StratigrajMcal Position. — I have collected this Urchin only in the Sponge- 

 gravel beds of Lower Greensand at Coxwell, near Faringdon, Berks. The fine specimen, 

 fig. 1 a, was obtained from this locality, and presented to me by my old esteemed friend 

 Thomas Davidson, Esq., F.R.S. I am likewise indebted to E. C. Davy, Esq., F.G.S., 

 Wantage, for several specimens more or less perfect to complete my description of 

 the anatomy of the test of this very rare form. It is worthy of note that the group to 

 which I refer the species all come from beds appertaining to the Middle Neocomian, 

 "Etaye Neocomien vioyen," a fact of importance helping to determine the age of the 

 Sponge-gravel beds of Berkshire. 



Genus — Caratomus, Ayassiz, 1840. 



Small Urchins Avith an ovoid or circular test rounded before and often rostrated 

 behind ; the sides are thick and inflated, the upper surface is convex, and the apical disc 

 excentral and forwards ; the base is convex with a slight depression arftuud the mouth- 

 opening, which has neither lobes nor pores. 



The vent is infra-marginal and not visible from the upper surface, it is transversely 

 oblong or triangular, and situated in several species in a rostrated development of the 

 single inter-ambulacrum. 



The ambulacra are short and subpetaloid, and the zones are formed of simple, equal, 

 non-conjugate pores, disposed in pairs,which are closely approximated at the summit, apart 

 in the middle of the zone, and approximated at the ambitus, PI. LVII, fig. 2 h, c ; at 

 the base they are feebly indicated by lines which converge around the mouth-opening. 



The tubercles, which are large for so small a test, are scrobiculated, those at the base 

 are the largest, and the sm-face of the plates likewise is covered with minute granu- 

 lations. 



