296 



THE ECHINOIDEA 



ambulacra then form the " trivium," and the two posterior form 

 the "bivium." This may be clearly seen in the common Chalk 

 Echinoid, Ecliinocorys scutatus, Leske (syn. Anancliytes ovatus, Lam.), 

 (Fig. XXXV. 2). In some cases the separation of the trivium and 

 bivium becomes greater ; it is finally completed in the Jurassic 

 genus Collyrites, and the living genus Pourtalesia, in which the 

 apical system is broken up into two parts, separated by a zone of 

 ordinary interambulacral plates. 



Important changes also take place in the plates of the test, 

 affecting both their structure and arrangement. 



The Interambulacral Plates are biserial in the great majority 

 of Echinoidea, but they may be uniserial, triserial, or multiserial. 

 Each plate may bear one tubercle or more than one ; to increase 

 the strength of the muscular attachment of the spine, the mamelon 



4 FIG. XII. 5 



Types of ambulacral plates. 1, cidaroid ; 2, diademoid ; 3, arbacioid ; 4, echinoid ; 

 y, cypliosomout. d, demiplates ; p, primary plates. 



is perforated by a small hollow, and the boss becomes irregular, 

 owing to a series of crenulatioris. 



Thus among other variations of interambulacral plates are the 

 following : Those which are unituberculate with the tubercles 

 either plain or crenulate, perforate or imperforate ; those which 

 are bi- or multi - tuberculate, and have the tubercles either 

 perforate or imperforate; those which are granulate, as in 

 Palceechinus ; those which have their edges bevelled, as in Echino- 

 thuria. 



The principal variation in the Ambulacral Plates is in the 

 number of elementary plates (shown by the number of pore-pairs) 

 in a compound plate. There are five main types : 



1. The cidaroid, when all the plates are low, simple primaries, 

 as in the Cidaridae, Orthopsinae (Fig. XII. 1). 



2. The diademoid, when all the plates are primaries, but they 

 are united in sets of threes into compound plates (Fig. XII. 2). 



