316 HOLASTER 



The apical disc is narrow and elongated ; its elements consist of four perforated 

 genital plates, on the right anterior of which the small raadreporiform tubercle rests 

 between the anterior and posterior pair ; two ocular plates are interposed; the single 

 ocular is seen at the summit of the single ambulacra, and the two posterior oculars at 

 the summits of the hinder pair ; the elements of the disc are closely united together, 

 and the surface of the plates covered with fine granules. 



The vent is large, and opens at the upper part of the anal area immediately below 

 the carina (fig. 1 e). The periprocte is elliptical, with a well-defined margin accumi- 

 nated at both extremities, and having the lower part of the border hollowed out 

 (fig. 1 e). 



The mouth is transversely oblong, placed near the anterior border in a depression 

 formed by the anteal sulcus. The peristome is transversely oblong (fig. 1 I/), and is 

 rounded before, with a thickened projecting lip behind. Around the aperture are several 

 pairs of round ambulacral pores arranged in a stellate figure (fig. 1 d). 



Affinities and Differences. — This Urchin was long a puzzle to palaeontologists, and we 

 are indebted to d'Orbigny for having removed the confusion that existed, and for 

 proving that Defrance's Urchin figured by Brongniart was the true type of the species. 

 The English specimens are small, and resemble the Ui'chin described as Holaster Cenoma- 

 nensis, d'Orb., which, however, is only a small variety of Hoi. suborbicularis. M. 

 Cotteau is disposed to unite this species to Cardiaster fossarius, Bennet, an opinion in 

 which I cannot concur, as I feel satisfied that, if my learned friend had before him good 

 type specimens of Card, fossarius, he would readily distinguish the differences between 

 that Urchin and llol. suborbicularis. Putting aside the question of the lateral fasciole, 

 which I have never seen in Card, fossarius, the other features of the test are sufficiently 

 distinct to show the differences which exist between them (see PI. LXVIII). 



In Holaster fossarius the shell is shorter and broader than in Hoi. suborbicularis. 

 The anteal sulcus is deeper, and retains its depth from the mouth to the disc, whereas 

 in Hoi. suborbicularis the sulcus vanishes above the anterior border, and leaves the upper 

 surface smooth and undepressed. The carinas on each side of the sulcus are more 

 prominent and much sharper in Hoi. fossarius than in Hoi. suborbicularis, and the 

 avenues of pores in the antero-lateral ambulacra are wider apart and more petaloid ; 

 whilst the base is flatter and the plastron less prominent in Hoi. fossarius than those 

 parts are in Hoi. suborbicularis. 



Locality and Stratiyraphical Position. — This Urchin abounds in the bed of Chalk 

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

 found likewise plentifully in the Chalk-Marl itself, and more rarely in the Lower Chalk. 

 It has been collected in nearly all the English Counties in which these beds are found, 

 but has not been observed in Ireland. 



Tlie Foreign Distribution. — Villers-sur-Mer, Vaches-Noires (Calvados) ; Montague 

 St. -Catherine pres Roucii ; Fecamp (Scine-Inferieure) ; Fourneaux, la Madelaine 



