80 CLASSIFICATION OF THE. 



Family 2. — HEMiciDARiDiE. (Not yet found in British Cretaceous strata.) 



Family 3. — Diademad^. 



This Family includes large and small Urchins having a thin, circular, pentagonal, and 

 subpentagonal test, more or less depressed on the upper surface, and fiat at the base. 



The ambulacral areas are wide and straight, with two rows of primary tubercles, often 

 as large and numerous as those of the inter-ambulacral areas. 



The poriferous zones are narrow, almost always straight, and sometimes subflexiious ; 

 the pores are unigeminab bigeminal, and trigeminal in their arrangement in different genera. 



The inter-ambulacral areas are in general twice the width of the ambulacral, and occu- 

 pied, at the equator, with two, four, six, or eight rows of primary tubercles, Avhich diminish 

 gradually in number near the poles. The bosses of all the tubercles are small ; their 

 summits, in general, are crenulated, sometimes uncrenulated ; the tubercles are small, in 

 general perforated, in Cyphosoma imperforate ; they are in general a little larger than those 

 of the ambulacra; but are often of equal magnitude in both areas. 



The apical disc is small, and situated opposite to the mouth ; it is composed of five 

 ovarial and five ocular plates ; the anterior pair of ovarial plates are a little larger than 

 the posterior pair, and the right antero-lateral plate, with a small, spongy, madreporiform 

 body on its upper surface, is the largest ; the vent is round or oblong, and generally in 

 the centre of the disc ; the ocular plates are very small, and distinguished with difficulty. 



The mouth-opening is in general large and decagonal, and the peristome divided into 

 ten lobes by deep notches ; the jaws in general are large and powerful. 



The spines in existing genera are long, slender, and tubular, sometimes three 

 times as long as the diameter of the test.-' In the fossil extinct genera they rarely attain 

 the length of the diameter of the test, and are short, stout, and solid, except in Hemipcdina, 

 which have long hair-like spines. The long tubular spines of living Diademas, and a rare 

 form from the Cretaceous rocks, are encircled by spiral verticellate processes, or fringe- 

 like scales, PL XIV, fig. 2, whilst the surface of the solid spines of Pseudodiademas is in 

 general covered with fine longitudinal lines ; neither prickles or asperities being developed 

 on their stems, 



Lamarck divided the genus Cidaris of Klein into two sections, " Les Turbans' and " Les 

 Diadhnes /' these were afterwards by Dr. Gray^ erected into genera ; the Cidaris radiaia, 

 Leske, constituting a third type, formed his new genus Astropyya. The genus Cidarites of 

 Lamarck was considered to form a natural family, including the genera Cidaris, Biadema, 

 and Astropyya, which he constituted and characterised thus : — 



1 Peters, ' Ueber Gruppe der Diademen,' p. 2, 101. Konigl. Akademie der Wissencliaften Augt., 

 1853, Berlin. 



^ 'Annals of Philosophy,' new series, vol. x, p. 426, 1825. "An attempt to divide the Echinidse or 

 Sea-Eggs into natural families." 



