FROM THE WHITE CHALK. 279 



This structure of the ambulacra forms a good diagnostic character between this species 

 and M. cor-anguinum. 



The apical disc is very small, and lodged in a valley formed by the confluence of 

 the apices of the five ambulacra ; the madreporiform tubercle occupies the centre, sur- 

 rounded by four ovarial holes, the posterior pair being wider apart than the anterior 

 pair ; the culminating summit of the upper surface being immediately behind the disc. 



The periprocte is situated at the upper part of the obliquely truncated posterior border, 

 and its position is persistently the same in all the six specimens before me. 



The base is flattened, the portion between the peristome and anterior border is slightly 

 hollowed out. The plastron is wide and filled with large close-set tubercles, these radiate 

 in regular rows from a point rising near the middle of the fasciolar line ; the lateral 

 portions slope away to the ambitus ; the valleys between indicate the course of the basal 

 portions of ambulacra between the border and the mouth, which is situated at a short 

 distance from the anterior sulcus, and is bilabiate, and surrounded by a rosette of pores 

 and tubercles very regularly arranged in the small specimen figured in 3 b. 



Affinities and Differences. — Micraster breviporus, which has long been considered to be 

 a variety of M. cor-anguinum and of M. cor-testudinarium, is distinguished from them by 

 its outline, which is more elongated and depressed, tapering at the sides, and less 

 acuminated posteriorly; its upper surface is more uniformly convex, its ambulacral 

 summit is excentral and placed forwards, and its petaloidal ambulacra are shorter and 

 less depressed, and the ambulacral plates are smooth and not tumid as in M. cor- 

 anguinum. 



Micraster lawoporus, d'Orb., has the summit more central, the ambulacra much more 

 depressed, the anterior half of the upper surface more declined, and the posterior half 

 more strongly carinated. I have compared my specimens from Brighton and Norfolk 

 with a type-specimen kindly given me by my excellent friend M. De Loriol, which was 

 collected at St. Julien du Sault (Yonne) ; from the "etage Senonien," equal to our White 

 Chalk. M. De Loriol 1 says, "In 1837 M. Desmoulins gave the name of Spatangus 

 Leskei to the Urchin figured by Klein under the name Spatangus cor-anguinum, var. 

 Norvagicum and var. productum, which differs really from the true Spatangus cor-anguinum. 

 D'Orbigny and other authors have referred to this Micraster {Spatangus Leskei) the 

 species named by the late Professor Agassiz Micraster breviporus. Recently M. Hebert 

 having been able in Denmark to examine the original specimens of the species which 

 Klein had figured under the name Sp. cor-anguinum, var. Norvagicum, has recognised that 

 it differs in reality from Micr. breviporus, and that it ought to be separated, and continue 

 to bear the name Micr. Leskei, which appears to be a form special to the Upper Chalk 

 of the north of Europe. 



Locality and Stratigraphical Position. — My specimens of this species have been col- 

 lected from the Upper Chalk of Brighton, Sussex ; Balsham, Cambridge ; and from the 

 1 ' Paleontologie Suisse Echinides Cretaces,' p. 370. 



