302 Dr. T. Wright on the Cassidulidse of the Oolites. 



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

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

 distinguish iV. orbicularis from its congeners. The ambulacra 

 are nearly all of the same width ; they have a more petaloid form 

 than in the preceding species, in consequence of the pores being 

 set farther apart and connected by deeper sulci ; near the margin 

 of the test the pores become very small and approximated, on 

 the base they are very indistinct and more widely apart, and 

 continue so to the mouth. The interambulacra are unequal, but 

 less so than in N. clunicularis and N. dimidiatuSj and the single 

 area is the widest. The test is covered with microscopic tuber- 

 cles ; so minute in fact are they, that without the aid of a good 

 lens the surface appears quite smooth. The anal valley is nar- 

 row, and extends from the vertex to the margin. The base 

 is concave and slightly undulated from the convexity of the in- 

 terambulacra and the straightness of the ambulacra. The mouth 

 is excentral and slightly five-lobed. The apical disc is broad, 

 and formed of two pairs of large perforated and a single imper- 

 forate ovarial plate, having the madreporiform element occupy- 

 ing the centre thereof ; the size of the ovarial plates occasions a 

 greater distance between the ocular plates^ which are conse- 

 quently more apart than in the other species. The test is very 

 thin, and the spines, which are preserved in one individual, are 

 moderately long and needle-shaped. 



Affinities and differences. — The orbicular form and long anal 

 valley liken N. orbicularis to some individuals of N. sinuatus ; 

 from them however it is distinguished by the tumidity of the 

 sides and flatness of the dorsal surface ; it is known from N. Hugii 

 by the anal valley extending from the disc to the margin, whereas 

 in the latter species the upper anal border is separated from the 

 vertex by an undepressed portion of the shell ; from the other 

 Nucleolites of our Oolites it is very distinct. 



Locality and stratigraphical range. — It is found in the Inferior 

 Oolite of Rodborough Hill, the Great Oolite of Minchinhampton, 

 and Salperton Tunnel, Great Western Railway, and the Coralline 

 Oolite of Calne, Wilts. Mr. Phillips^s specimens were collected 

 from the Cornbrash near Scarborough ; it has likewise been found 

 at Wollaston near Wellingborough, Northamptonshire. 



History. — Figured by Mr. Phillips in his ^ Geology of York- 

 shire,' but not described in that work ; enumerated by Professor 

 Forbes in his note on " British Nucleolites " appended to the 

 description of the 9th plate of his 1st Decade*, and now de- 

 scribed in detail for the first time. 



Mem, of the Geol. Survey of Great Britain. 



