GLYPTICUS. 185 



Genus— GLYPTICUS, Agassiz. 1840. 



The urchins grouped in this small genus constitute a type which is easily recognised, 

 although in an organic point of view their most striking external character is not of the 

 first value ; in fact, the irregular structure of the tubercles of the inter-ambulacral areas 

 essentially determines the peculiar physiognomy of the genus. 



The small urchins composing this genus have a thick test, and a round, depressed, or 

 sub-conoidal form; the ambulacral areas are narrow and straight, with two rows of 

 marginal tubercles, very regularly arranged throughout. 



The poriferous zones are narrow, and the pores strictly unigeminal. 



The inter-ambulacral areas have two rows of well-developed tubercles at their base ; 

 but on the upper part of the area the regular tubercles disappear, and the surface of the 

 plates is deeply sculptured with remarkable figures, which, in some species, resemble 

 hieroglyphical characters. 



The apical disc is large ; the genital plates are prominent, and have their surface sculp- 

 tured ; the eye-plates are large, and the lines of division between the plates are very 

 strongly marked. 



The mouth opening is wide ; the peristome decagonal, the notches are shallow, and 

 the lobes very unequal. 



The tubercles of both areas are imperforate, and have smooth, uncrenulated bosses. 



Glypticus resembles Temnopleurus in having the plates sculptured on the upper surface; 

 but in the former the figures are in relief, whilst in the latter the sculpture consists of deep 

 impressions corresponding to a portion of the sutures of the ambulacral and inter-ambulacral 

 areas. In Temnopleurus the tubercles form regular rows, and are crenulated, but imper- 

 forate ; the poriferous zones are undulated, and the pores are in threes. Glypticus was 

 thought to resemble Cyphosoma, but I cannot detect any two characters in common in 

 these genera. In Cyphosoma the tubercles are regular, and well developed through- 

 out; they have distinct areolas, crenulated bosses, and imperforate summits; the poriferous 

 zones are undulated, and the pores bigeminal in the upper part of the zones. 



Glypticus is distinguished from Pseudodiadema in the irregular sculpture on the inter- 

 ambulacra, and in having the tubercles imperforate and uncrenulated. It resembles one 

 section of that genus, however, in having the pores unigeminal in the zones. 



Glypticus resembles Stomechinus in having the tubercles imperforate, and with smooth 

 bosses; but is easily distinguished by the sculpture on the plates, and the size and 

 foliated character of the apical disc. 



The species of this genus have been hitherto found only in the Oxford Clay, Coral Rag, 

 and Portland beds, so that it belongs to the Middle and Upper divisions of the Oolitic 

 group. 



