158 PELTASTES 



Pkltastes umbrella, Agassiz, MSS. PI. XXXIV, fig. 1 a — d, fig. 2 and 3. 



Salenia umbeella, Agass. MSS. Morris's Catalogue of British Fossils, p. 58, 1843. 



— — Id. Forbes, in Morris's Catal. of British Fossils, p. 89, 1854. 



— CLATHRATA, Id. Woodward (pars), Mem. of Geol. Surv., App. to Decade V, 



1856. 



Diagnosis. — Body subglobose, convex above ; apical disc large, covering the upper 

 surface, its outline deeply indented between the ovarial and ocular plates, sutural grooves 

 deeply and sharply cut, those connecting the centres of the plates forming a distinct pen- 

 tagon. Each oviductal hole forms a centre, from which five grooves radiate, having angular 

 pits between them ; sur-anal plate marked M'ith an inverted triangle formed by horizontal 

 and inclined incisions. Ambulacra wide ; two rows of mammillated tubercles crowded 

 with granules around the base ; interambulacra with four large tubercles in the upper part, 

 and all the others small, diminishing towards the peristome ; base flat, mouth-opening 

 small. 



Dimensions. — Height, one quarter of an inch ; latitude half an inch. 



Description. — I have figured the type specimen of this form belonging to the British 

 Museum, as it is the species Prof. Agassiz gave this manuscript name to many years 

 ago, when he examined the Cretaceous Urchins in the National Collection. Whether P. 

 umbrella is specifically distinct from Peltastes clathratus I am not in a position to decide. 

 The apical disc is certainly much more angularly incised, and more sharply defined than 

 in P. clathratus ; and placing two well marked specimens of these type forms in contrast, the 

 decision would be in the affirmative ; but then we have the evidence of M. Cotteau, who 

 informs us that the incisions and impressions on the disc alone are not to be relied on as 

 specific characters, as one form glides into another by a series of intermediate gradations, which 

 connect together forms that appeared when isolated to be very distinct from one another. 



The ambulacral areas are nearly straight, and have two marginal rows of small mam- 

 millated tubercles, fourteen in each. The mesial space between the rows and tubercles is 

 filled with microscopic granules (fig. 1 c). The poriferous zones are conspicuous, and the 

 pores placed in oblique pairs. The inter-ambulacral areas are wide, and the plates in 

 the columns unequally developed, those above the ambitus are the widest, and support 

 large tubercles ; from the ambitus to the peristome they gradually become smaller, and 

 their respective tubercles diminish in the same ))roportion (fig. 1 0, fig. 1 c, and fig. 2). 



The apical disc is large, and its circumference much indented between the margin of 

 the ocular and ovarial plates (fig. 1 a, and fig. 3), presenting two beautiful varieties of this 

 remarkable structure. The sur-anal plate situated before the periprocte is marked with 

 incisions that form a triangle ; each oviductal hole of the two antero-lateral ovarial plates 

 forms a centre, from which five incisions radiate, and from each oviductal hole in the other 

 three ovarial plates four incisions radiate outwards, having angular pits between 



