210 DISCOIDEA 



Localitij and Stratigraj)liical Position. — This species is extremely rare; on the 

 authority of Professor M'Coy one only has been found iu England in the Upper Greensand 

 of Cambridge ; and this type-specimen, with its anal plates, is in the Woodwardian 

 Museum of Cambridge. 



Tlie Foreif/n Localities, according to M. Cotteau, are Rouen (Seine-Inferieure), Neu- 

 ■chatel pres Boulogne (Pas-de-Calais) ; Verronnet (Eure) ; La Chapelle Saint-Aubin, Les 

 ]\Ienus pres la Loupe (Sarthe) ; environs de Villedieu (Loir-et-Cher), from the Etage 

 Turonien, where it is very rare. 



DiscoiDEA Favrina, Bcsor, 1842, PI. XLVIII, fig. 1 a — g. 



DiscoiDEA F.vvRiXA, Besor. Monogr. Jes Galerites, p. 62, pi. vii, figs. 12 — 16, 1842. 

 — — Forbes. Mem. of Geol. Survej-, Decade I, descrip. pi. viii, 1849. 



Diaf/noms. — Test sub-pentagonal ; upper surface elevated, round, more or less inflated ; 

 base flat ; mouth-opening small ; vent oblong, midway between the peristome and border ; 

 inter-ambulacra wide, two prominent rows of primaiy tubercles ; ambulacra narrow ; five 

 plates opposite one inter-ambulacral. 



Dimensions. — Height seven tenths of an inch ; latitude one inch. 



Description. — This Urchin was first figured by my friend Professor Desor under the 



name Discoidea rot/da; he informs us that when the plates were executed for his 



beautiful memoir on the Galeritida his knowledge of the D. rotula was limited to moulds 



of this species, or to moulds with a fragment of the test adherent, but so much effaced 



that it was impossible to study its intimate structure. As he had recognised among 



the Urchins sent to M. Agassiz by M. Alex. Brongniart from the " Glauconie" of Bouen, 



who had first figured D. rotula, a species very similar in form to the others, he thought he 



could identify it with D. rotula ; subsequently ^L Favre, of Geneva, sent from Saxonnet 



a specimen of D. rotula with its test perfectly preserved. This specimen he compared 



with those sent from Rouen, when he found that the tubercles on D. rotula from Saxonnet 



were very different from those on the specimens from Rouen, for instead of being 



scattered without apparent order on the surface of the test, they formed horizontal series 



very continuous, resembling those on D. macrojiyga. This discovery determined M. 



Desor to separate the Saxonnet specimen from those derived from the " Glauconie " of 



Rouen, and to describe it under the name Discoidea Favrina. 



The specimen I have figured from the British Musemu collection was identified by the 

 late Dr. Woodward as the representative of Desor's species from the Upper Greensand ; 

 the test has a subpentagonal outline, is considerably elevated with a convex dorsal surface 



