On Cyrtoma, a new genus of Fossil Echinida. 185 



tions. The matrix in which the first were imbedded con- 

 sisted of fine sand with layers of dark paste, like the 

 alternate deposits that take place in estuaries. The Echi- 

 nida, on the other hand, are imbedded in coarse sand, 

 as well as in a greenish grey friable sandstone, which 

 will probably prove to be equivalent to the green saliferous 

 marls of the upper new Red Sandstone,* which here forms 

 the most recent and superficial deposit next below the 

 vegetable mould of the district. In future papers I hope to 

 describe most of the more perfect specimens I have been able 

 to collect during my hasty visit to Cherra Ponji, but I must 

 regret the want of a suitable opportunity of following up this 

 interesting discovery in a manner worthy of its importance. 



Genus— CYRTOMA,f J. M. 



Disc oval and thin, arched to the apex ; ambulacra petaloid, 

 and either broad and flat, or more elevated, and placed on narrow 

 ridges radiating from the apex to the disc. The two posterior 

 ambulacra are closer together than the others, with an inter- 

 mediate dorsal ridge leading to a dentated anus, and a depression 

 or hollow between the latter and the disc. Inferior surface flat, 

 mouth small and central, with five clavate ambulacra prolonged 

 to margin. 



Obs. All fossil from the Cherra Ponji beds. 



Note. — They are nearly allied to the genus Clypeus, Klien and Agassiz 

 (Echynoclypeas, De Blain,) which are all fossil from the jura, chalk, and tertiary 

 deposits. 



1. Cyrtoma herscheliana, J. M. t. v. figs. 1. 2. 3. 



Apex either symmetrical or oblique, J and equal in altitude to 

 about one-third of the shortest diameter; ambulacra converging, 

 lanceolate, with five rows of holes in each. The anterior ambu- 

 lacrum extends to the disc, the others to about half way between 

 the apex and disc, to which last they are each prolonged by a 

 narrow ridge. 



* Vide our remarks on Mr. Murchison's Silurian System, page 21. 



f Etym. from KtorW|tta, that which is bent or convex, a hump or a tumour. 



% This irregularity in some of the specimens has been doubtless occasioned 

 by pressure, to which these animals were suddenly exposed in a fresh and plastic 

 state, since the dead crust of Echinida becomes too brittle and unyielding after 

 death to bear pressure without breaking. 



