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DISCOIDEA 



Locality and Stratigraphical Position. — This species is extremely rare ; on the 

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

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

 Museum of Cambridge. 



The Foreign Localities, according to M. Cotteau, are Rouen (Seine-Inferieure), Neu- 

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

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

 Turonien, where it is very rare. 



Discoidea Eavrina, Desor, 1842, PI. XLVIII, fig. 1 a—g. 



Discoidea Favrina, Besor. Monogr. des Galerites, p. 62, pi. vii, figs. 12 — 16, 1842. 

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



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

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

 inter-ambulacra wide, two prominent rows of primary 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 rotula ; 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 Rouen, 

 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 M. Eavre, 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. macropyga. 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 Museum 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 



