322 HOLASTER 



The apical disc is much elongated (fig. 1 a, f) ; it is composed of four perforated 

 genital plates ; the right anterior plate has its surface covered with the madreporiform 

 body, and is separated from the posterior pair by two of the perforated ocular plates, 

 which are interposed between the anterior pair, and produce the length and narrowness of 

 the disc ; all the elements of the disc are closely soldered together, and densely covered 

 witli microscopic granulations, a few solitary tubercles only are seen in their midst. 



Where the anteal sulcus curves round the ambitus (fig. 1 e) a depression is formed 

 at the base (fig. 1 b), in which the mouth is situated ; the peristome is a small, trans- 

 versely oval, bilabiate aperture ; the upper lip is thickened and higher than the lower, 

 and the buccal extremities of the poriferous avenues form an irregular stellate figure 

 around the peristome (fig. 1 b). 



The posterior border of the test is truncated obliquely downwards and inwards 

 (fig. 1 c), and in some specimens slightly hollowed into a concave channel below 

 the vent, which occupies the upper part of the anal area (fig. 1 d), in a position about 

 half the height of the test. The periprocte is vertically of aa elliptical figure (fig. 1 d), 

 and its upper angle forms the termination of the dorsal carina which extends backwards 

 from the disc to the border. 



Affinities and Differences. — This Urchin is so well characterised by its specific 

 features that it is not easily mistaken for any other. Hoi. cor-avium, Laralc, has been 

 ascertained by d'Orbigny's inspection of the type in the museum to be a siliceous mould 

 of Hoi. subfflobosus ; and Hoi. nasutus, labelled by M. Agassiz in the museum, is only a 

 specimen of Hoi. subt/lobosus deformed by pressure. 



Locality and Straii(jrapldcal Position. — This Urchin abounds in the bed of Chalk 

 with green grains intervening between the Upper Greensand and Chalk-Marl. It is 

 found also plentifully in the Chalk-Marl itself and more rarely in the Lower Chalk. I 

 have had many fine specimens from the Chloritic Marl near Chard, and from the Chalk- 

 Marl near Lewes, in Sussex. The Chalk-Marl of Wiltshire and of Dorsetshire has yielded 

 many specimens. This species, says Forbes, has not been found in L'eland. 



Foreign Localities, — It is found at the Mountain Sainte-Catherine, near Rouen, 

 Fecamp, and Havre, at Saint-Parre and Laubresel, near Troyes, Aube ; with 

 Acantlioceras Tiotomagense, Brong., at Pourraiu, and Seigneley, Yonne ; Sancerre, Cher ; 

 Cassis, Bouches-du-Rhone ; Bidart, near Biarritz, Basses-Pyrenees ; Villers-sur-Mer, 

 Calvados ; Trinite, near Nice. In Switzerland, in the ' Etage Cenomanien,' Monsieur 

 De Loriol records it from Sainte-Croix, Vaud. ; Cheville, Valais ; Ried, near Bienne, 

 Berne ; Summit of Sentis, Ebenalp, Sentis ; Seealp, Appenzell, Yberg-Sil, Schwytz ; 

 Lac Saint-Point, Jura. In Germany, Goldfuss records it from Quedliuburg, Prussia ; 

 Neuwallmoden and Langelsheim, Brunswick. 



