212 Dr.T. Wright on the Cassidulidae of the Oolites. 



but are larger below, and are surrounded by a circular depression ; 

 the vertex is excentral, behind which the apical disc is situated ; 

 it is nearly central in some individuals, and quite so in others, 

 and is formed of two pairs of perforated ovarial plates, and a 

 single imperforate plate ; the madreporiform body occupying the 

 centre of the disc, and the five small ocular plates are seen at the 

 apices of the ambulacra. The base is more or less concave, and 

 deeply so at the point where the mouth-opening is placed ; this 

 aperture has a pentagonal form, and is much nearer the anterior 

 than the posterior border. The ambulacra form valleys that are 

 scarcely perceptible in passing from the border to the mouth. 



Affinities and differences. — In its general outline N. dimidiatus 

 resembles N. clunicularis, but is distinguished from that species 

 by its ovate anal valley, which extends only two-thirds of the 

 length of the dorsal surface of the single interambulacrum, 

 whereas in N. clunicularis it reaches from the apical disc to the 

 margin; the tumidity of the sides and posterior lobes afford 

 additional diagnostic characters. It is so entirely distinct from 

 the other Oolitic congeneric forms, that it cannot be mistaken for 

 either of them. 



Locality and stratigraphical range. — This species has been col- 

 lected from the Coralline Oolite of Calne and Steeple Ashton, 

 Wilts, and of Malton and Filey Bridge, Yorkshire ; it is found 

 likewise in the same stage in Oxfordshire and near Havre. 



History. — First figured as a species distinct from N. clunicu- 

 laris by Mr. Phillips in his ' Geology of Yorkshire/ and now de- 

 scribed in detail for the first time. 



Nucleolites orbicularis, Forbes. 



Syn. Clypeus orbicularis, Phillips, Geol. of Yorksh. vol. i. pi. 7. fig. 3. 

 Nucleolites orbicularis, Forbes, Mem. of Geol. Survey, Decade 1. de- 

 scription of pi. 7. 



Test orbicular, dorsal surface irregularly convex and depressed ; 

 sides tumid ; vertex and apical disc central ; ambulacra lanceo- 

 late ; anal valley narrow, extending from the disc to the border; 

 posterior lobes obsolete; base flat, slightly concave; mouth 

 subcentral, situated in a depression. 



Height y^ths of an inch, antero-poaterior diameter 1 inch and 

 ^th, transverse diameter 1 inch and 2 3 T) ths. One large specimen 

 is 1 J inch in diameter, but its proportional dimensions cannot be 

 ascertained, as it is crushed. 



Description. — The orbicular circumference, tumid sides, obso- 

 lete lobes, and broad, flat, and somewhat irregular dorsal surface 



