76 CIDARIS 



"Additional Note on CIDABIS PEROBNATA, Porbes. (See p. 62.) 



" This Cidaris is the largest of all the Cretaceous CiDARiDyE ; portions of a full-grown 

 specimen now before me, containing four complete columns of plates in contact, give the 

 following dimensions for the test — height, two inches and two tenths ; transverse diameter, 

 two inches and one tenth. The spines, like the body, also exceed those of all other 

 species. In a mass of spines of C. perornata from my cabinet, which are all one tenth 

 of an inch in diameter, is one which, although deficient of a portion of its apex, measures 

 in the remaining part of its length four inches and six tenths — this length is by no means a 

 maximum. The number of the plates and the form of the spines appear to have rendered 

 perfect examples of the test with spines attached exceedingly rare. Separate plates and 

 groups of broken spines are plentiful ; complete columns of plates uncommon. Small 

 Ostrea are occasionally found affixed to the spines. 



"The test, when full-grown, has, in the am bulacral areas, eight rows of granules at the 

 ambitus ; of which rows the two exterior are the largest and most evenly disposed, the 

 six interior are more numerous, of less size, and not so regularly arranged ; at the mouth- 

 opening there are six rows, at the anal four ; the second discal plate has nineteen pairs of 

 pores in the poriferous zone ; the proximal discal plate in each column has a rudimentary 

 tubercle and an elongate obsolete areola. The granules of the miliary zone are of two 

 sizes, the smallest of which occupy the spaces between the largest. In specimens of the 

 test of the usual size the first, second, and third of the plates, reckoning downwards from 

 the anal opening, have the upper half of the boss crenulated. The spines belonging to 

 the granules of the scrobicular margins are flat and somewhat fan-shaped; they are 

 covered with minute striae, which converge from the circular base (in which there is an 

 acetabulum) towards the smaller apex ; length two tenths of an inch, greatest width one 

 twentieth. The jaws of a full-grown specimen do not greatly differ in outline from those 

 of other species ; they are half an inch in length. 



" Cidaris perornata is tolerably common in the Upper Chalk ; it appears to commence 

 (where it is rare) in the middle of the flinty Chalk. 



"Additional Note on CIDABIS DIXONI, Cotteau. (Seep. 67.) 



" All the spines of this species hitherto found are of considerable size, and are clavi- 

 form, and inflated ; the apex is acute ; the surface covered with numerous granules, which 



