276 MICRASTER 



perfectly characterised by its sub-undulated ambitus, subconvex upper surface, its central 

 ambulacral summit, ambitus much grooved by the single ambulacrum, and its narrow 

 ambulacra deeply depressed. The 3Iicraster Bronpiiarti appears to be special to the 

 Chalk at Meudon, as but one mould has been collected near Sens." 



" 4th. The Micraster cor-angidnum, easily recognised by its cordiforra outline, dilated 

 before, acuminated behind, by its elevated and inflated upper surface, convex and rostrated 

 in the posterior region, with narrow and shallow petaloid ambulacra ; its ambulacral 

 plates shorter and deeper than in its congeners, and by its excentral apical disc placed 

 in the direction of the posterior border." 



Locality and StratigrapUcal position. — It is found in all the Upper Chalk districts of 

 England, and is common in Kent, Sussex, and Norfolk. It was recorded by General 

 Portlock from the Chalk of Magilligan, County Derry, Ireland. 



Toreign Distribution. — This Urchin is one of the most characteristic of the Upper 

 White Chalk, equivalent of the ' 22e Etage Senonien,' d'Orbigny. According to that 

 author it has been collected in the Anglo- Parisian basin at Meudon (Seine-et-Oise) ; 

 Beauvais (Oise) ; in the Somme ; in the Ardennes at Retheil ; in the Seine-Inferieure^ 

 at Ciqueport, Etretat, Fecamp, Dieppe, Treport; in the Yonne at Sens, Seigneley, 

 Joigny, Villeneuve-sur- Yonne ; in the Eure, at Chateaudun ; in the Sarthe, at Saint- 

 Erambault, Roches, Vendome ; in the Loir-et-Cher, at Couture, Villiers, Blois ; in 

 I'Indre-et-Loire, near Tours. 



In the Pgrenean basin at Perigueux (Dordogne) , at Moutiers, Cognac (Charente) ; 

 at Mirambeau, Coze, Meschers, Saintes (Charente-Inferieure) ; at Tercis, Riviere, near 

 Dax (Landes) ; at Mauleon, Magnoac (Hautes-Pyrenees) ; at Bidart (Basses-Pyrenees). 

 In the Mediterranean basin at Soulage, Songraigne (Ande) ; at Ayglun, Mers, Beausset 

 (Var) ; and in Scandinavia, Germany, Switzerland, and Belgium in the same geological 

 horizon. 



Micraster cor-bovis, Forbes, 1850. PL LXII a, figs. 1, 2 a — d. 



MicEASTER COR-BOVIS, Forbes. In Dixon's Geol. of Sussex, pi. xxiv, figs. 3, 4, p. 342, 



1850. 



— — Forbes. Mem. Geol. Surv., decade iii, note, pi. x, 1850. 



— _ Forbes. In Morris' Cat. of Brit. Foss., 2 ed., p. 83, 1854. 



— — Besor. Synopsis des Ecbinides Foss., p. 367, 1858. 



Diagnosis. — Test large, ovate, slightly cordate, broadest in the region of the antero- 

 lateral ambulacra. Posterior border obtusely subtruncated. Dorsal surface depressed, 

 and slightly elevated in the anterior region above the rest of its surface. Apical disc 

 excentral, nearer the anterior border. Anteal sulcus shallow, petaloid ambulacra short, 

 unequal in length, and slightly depressed. Base convex, vent in the middle of the 

 obtusely sub-truncated border. 



