2G4 



HOLECTYPUS. 



The Rev. A. W. Griesbach has collected the finest specimens I have seen in 

 the Cornbrash near Rushden, Northamptonshire ; the beautiful specimens fig. 1, a, and b, 

 are two of these, kindly presented for this work. The Cornbrash specimens are always 

 larger and finer than those collected from the Inferior Oolite. Mr. Gavey obtained it from 

 the Cornbrash near Woodstock, Oxon. ; and it is found abundantly at Scarborough, in 

 the Cornbrash near the Castle Hill. 



Holectypus HEMisPHiERicus, Agassiz. PI. XVIII, fig. 2 a, b, c, d, e,/,f/, h, i. 



Discoidea HEMisPHiEKiCA. Agassiz, Catalogus Systematicus, p. 7. 



— — Desor, Monographic des Galerites, pi. 8, figs. 4 — 7, p. 71. 

 Holectypus hemispilericus. Agassiz et Desor, Catalogue raisonne des Echinides, Annal. 



des Sc. Nat., 3 rae serie, vol. vii, p. 146. 

 Galerites hemispii^ericiis. Forbes, Memoirs of the Geological Survey, Decade III, 



pi. 6. 

 Discoidea marginalis. M'Coy, Annals and Magazine of Natural History, 2d ser., 



vol. ii, p. 413. 

 Holectypus Devauxianus. Cotteau, Etudes sur les Echinides Fossiles, pi. 2, figs. 7 — 9, 



p. 46. 



— HEMisPHiERicus. Wright, Annals and Magazine of Natural History, 2d ser., 



vol. ix, p. 96. 



— — Forbes, in Morris's Catalogue of British Fossils, 2d edit., 



p. 82. 



— — Desor, Synopsis des Echinides Fossiles, p. 172. 



Test hemispherical, more or less depressed ; sides tumid ; margin rounded ; posterior 

 half of the test longer than the anterior ; single inter-ambulacrum slightly produced ; 

 anal opening marginal, large, and pyriform, with the apex directed upwards ; base nearly 

 flat ; mouth opening small. 



Dimensions. — Height, eight tenths of an inch ; transverse diameter, one inch ; antero- 

 posterior diameter, one inch and a quarter. 



Description. — The marginal vent of this urchin sufficiently distinguishes it from its 

 congeners. It is very abundant in some localities, but rare in others ; it appears to have 

 had a very limited life in time, having been hitherto only found in a marly vein, about 

 an inch in thickness, which traverses the Trigonia bed of the Inferior Oolite, in the zone 

 of Ammonites Parkinsoni, Sow. I have examined many hundreds of specimens of this 

 Echinite, which all came from the same bed. 



M. Desor has given a very good figure of this species in his ' Monograph on the 

 Galerites,' which appears to have been the small conical variety. 



