314 



HOLASTER 



Affinities and Differences. — This species resembles, in many traits of structure, 

 Holaster Imvis. It is readily distinguished from that species by the ambulacral summit 

 being much more excentral, which occasions a much longer slope between the apical 

 disc and the posterior border, and a greater obliquity of the upper surface of the test ; 

 the anterolateral ambulacra are shorter and straighter, and the postero-lateral pair 

 much longer. The anteal sulcus is likewise shallower, and the posterior border much 

 narrower. The rotundity and smoothness of the test, and the width and shallowness of 

 the anteal sulcus form a marked difference between it and Holaster suborbicularis, and 

 the same characters serve to separate it very distinctly from Holaster subglobosus. 



Locality and Stratigrapliical Position. — This Urchin appears to be rare. I have only 

 seen three specimens ; two of these were collected from the Lower Chalk near Folkestone, 

 and the third was obtained from the Upper Greensand at Chute Farm, near Warminster, 

 Wilts. I have compared this Urchin with a large number of Holasters from the Chalk 

 of Prance and Switzerland, in the cabinet of my friend M. De Loriol; and it is 

 easily distinguished from all its congeners by the excentral position of the apical 

 summit, and the long sloping declension of the posterior two thirds of the upper surface. 

 The specific name excentricus was at first suggested for this species, but as we have the 

 Infulaster excentricus from the Cretaceous Rocks it was better to adopt obliquus as 

 indicative of the obliquity of the upper and posterior surfaces of the test, in order to avoid 

 any confusion regarding this new form, which I have now described and figured for the 

 first time. 



Holaster suborbicularis, Defrance. PI. LXXIV, figs. 1 a — e. 



Spatangus suborbicularis, Defrance. MS., 1821. 



— — Brongniart. Descrip. geol. Paris, p. 85, pi. v, fig. 5, 



1822. 



— — Blainville. Mem. d'Actinologie, p. 204, 1834. 

 Holastee — Agassiz. Prodrom. d'une Monogr., p. 16, 1836. 



— — Desmoulins. Etudes sur les Ech. Foss., p. 400, 1837. 



— — Deshayes, in Lamarck, 2nd ed., An. sans Vert., t. iii, 



p. 334, 1843. 



— — Sismonda. Mem. geol.-zool. Echin., p. 8, 1843. 



— — d'Orbigny. Paleontol. Franc,., Ter. Cretaces, t. vi, p. 93, 



pi. 814, figs. 6, 7 ; pi. 815, 1853. 

 d'Orbigny. lb., p. Ill, pi. 819, figs. 7—12, 1853. 

 Desor. Synopsis des Echin. Foss., p. 340, 1858. 

 Cotteau. Echinides de la Sarthe, p. 198, pi. xxxiii, 



figs. 1—6, 1860. 



CENOMANENSIS, 

 SUBORBICULARIS, 



Diagnosis.— Test oblong, cordiform, rounded and deeply grooved before, contracted 



