3:24 HOLASTER 



areas and on other parts of the test, and large tubercles, developed upon the inter- 

 ambulacral plates without much order or regularity, and varying much in number in 

 different specimens. These tubercles are perforated at the summit, and raised upon 

 bosses with crenulated rims. The specimen I have figured shows this tubercular 

 structure to perfection, but the majority of specimens want the profusion of large 

 tubercles seen on the figured shell (PI. LXXIV, fig. 2). Besides the tubercles just 

 described, the surface of the plates is covered with microscopic granules which form 

 circles around the base of the tubercles and crowd the spaces between them with a close- 

 set granulation. It is only in well-preserved specimens, however, that this structure 

 can be observed, and then only by the assistance of a lens. 



Affinities and Differences. — This fine Urchin was well figured by the late Prof. 

 Leymerie in 1842, 1 and afterwards by d'Orbigny in 1S53. 2 I had been fortunate in 

 procuring a very fine example of this Holaster from the Grey Chalk of Folkestone, and I 

 desired my late friend Mr. Bone to make most accurate figures of it, which he has done, 

 and given five beautiful drawings of the same in Plate LXXIV, fig. 2 a — e. 



At first sight one naturally concludes that Hoi. Trecensis, Ley., is a well-defined form, 

 and fully entitled to specific rank, but a closer examination of the test, and a comparison 

 of it with other fossils considered to be Hoi. carinatus, weaken our faith in this belief. 

 Characters at first sight thought to be specific appear to be evanescent. The size and 

 number of the tubercles on the upper surface of Hoi. Trecensis, for instance, are not 

 constant, seeing that in some specimens they dwindle away, and in others almost dis- 

 appear. Finally, the close identity of Hoi. Trecensis with Hoi. carinatus, from being a 

 doubt, becomes a certainty. Now arises the next question, in what does Hoi. carinatus 

 differ from Hoi. Icsvis ? Failing to discover the specific differences which many eminent 

 palaeontologists have thought they saw in these different forms, we have at length arrived 

 at the conclusion that my learned friend Mons. De Loriol came to, after a patient and 

 exhaustive study of Hoi. Icsvis, namely, that these three forms — Hoi. carinatus, Hoi. 

 Trecensis, and Hoi. Icsvis — are only varieties more or less defined of Hoi. I avis, De Luc. 



Locality and Stratigrapliical Position. — The fine specimen figured in PI. LXXIV, 

 fig. 2, was collected from the Grey Chalk, near Folkestone, and I have several other 

 large specimens from the Lower Chalk of Wiltshire and from the Medial Chalk, 

 Swaffham, Norfolk. 



Foreign localities. — Prof. Leymerie found his type specimen at Saint-Parre, near 

 Troyes, Aube ; it has been collected likewise at Mont Saint-Catherine, Rouen, Seine- 

 Inferieure, with Acanthoceras Bhotomagense ; and from the Chloritic Chalk in the 

 environs of Saint-Croix, Vaud, Switzerland. 



1 'Mem. de la Soc. geol. de France,' p. 5, pi. ii, fig. 1, 1842. 



2 ' Paleontologie Francaise, Ter. Cretaces,' torn, vi, pi. 847, 1853. 



