FROM THE WHITE CHALK. 



839 



MiCRASTER BREVis, Dcsor. PI. LXXV, figs. 2 </, 3 a — -y. 



MiCRASTER LATUS, 

 BREVIS 



Sismonda. Echin. foss. de Nizza, p. 29, tab. i, fig. 13, 18-13. 



Hebert. Etud. sur les terr. Cretaces ; Mem. Soc. Geol. de 

 France, 2e ser., torn, v, tab. xxix, fig. 19, 1854. 

 Spatangus ananchytoides, Z)e««0!<^ Tab). Synom., p. 406, 1837. 

 MiCRASTER BREVIS, Besor. Synopsis des Echinides Foss., p. 364, 1858. 



Diagnosis. — Test cordiforiu, very short, as broad as it is long ; antero-lateral ambulacra 

 long and wide, postero-lateral pair shorter ; poriferous zones wide, boles forming a pair 

 placed far apart.. Test elevated, sometimes conoidal ; anteal sulcus very shallow ; single 

 ambulacrum similar in structure to the anterior pair ; posterior border narrow and very 

 obliquely truncated. 



Description. — This Urchin appears to belong to the " Cra?e « /^^jownVes " and has 

 ixany structural affinities with certain forms of M. cor-testudinarium ; it is, however, a 

 much wider shell with a more elevated upper surface, and as far as external form goes 

 very different from that species ; the summit is excentral, a little nearer the anterior 

 border. The ambulacral areas are built of narrovi' elongated plates, ^nd the pores 

 forming a pair are placed wider apart than in any of the congeneric forms I have 

 described. 



My late friend, Monsieur Michelin, sent me several very good type shells from the 

 " Craie a hippurites " from Sougraigne, near les Bains de Rennes, and Soulage, Cor- 

 bieres, Aude ; it is found also at Tuco, near Caseneuve, " Craie de Tercis au Perigord 



