ECHINODERMATA. 11 



the border. During the life of the animal this opening was closed by an anal membrane, 

 and a series of small angular anal plates, the number and disposition of which vary in 

 the different genera. The anal plates are seldom preserved in fossil species ; and the term 

 anal opening is given to all that part of the test occupied by them, the anal membrane, 

 and the vent. 



The Amhulacral and Inter-ambulacral Areas. 



The test is composed — 1st, of twenty columns of calcareous plates of different sizes, 

 the plaquettes, Tdfekhen, Jssula, of authors. They are pentagonal in form, and are united 

 by harmonial sutures to form rays, which proceed from the mouth, where they have their 

 greatest breadth, to the apical disc, where they are narroM^est. 2d, of a series of hexagonal 

 or polygonal plates, forming a disc, which occupies the upper surface of the test. 3d, of 

 ten rows of small plates, which are notched on their margins, to form holes : these consti- 

 tute the poriferous zones. 4th, of moveable spines, which are jointed with eminences on 

 the outer surface of the columnar plates. 



The amhulacral plates form two narrow columns, which are bounded by two poriferous 

 zones. The space thus circumscribed is the amhulacral area. There are five of these areas 

 in the test of the Echinoidea; in the Cidarid^ the amhulacral areas are very narrow, 

 and support only granules (PI. II, fig. \a,h); but in the Echinid^ (PI. VIII, fig. 1) 

 they are much wider, and have large tubercles on their surface. The comparative width of 

 the ambulacra, as compared with the inter-ambulacra, has led some authors* to divide the 

 family Cidaridse, including therein the Echinidse, into two tribes, — the Angustistell^e, 

 or Cidaridse with narrow ambulacra, and the LATiSTELLiB, or Cidarida3 with broad 

 ambulacra. These two tribes nearly represent our two families : the Cidarid^ are equal 

 to the Angustistell^, and the Echinid^ are nearly equal to the Latistell^. 



PI. I, figs. 1 ^, 2 c, 3 c, 4 c; PL II, figs. 1 c, 2^7 represent the structure of the ambu- 

 lacra in the Cidarida ; and PL VI, fig. 2 d, and PL VII, figs. 3 and 4, show the structure 

 of the amhulacral areas in some Echijiida. 



One of the amhulacral areas is single, and always represents the anterior region of 

 the test. In the spheroidal Ecliinidce and Salenida, this is detected by its relation to the 

 apical disc, as the right antero -lateral genital plate always carries the madreporiform body ; 

 but in the oval, pentagonal, and elongated forms, its position and relation to the mouth 

 render it unmistakeable. The four other ambulacra are disposed in pairs. 



The inter-ambulacral plates form two broad columns, which compose the inter-ambu- 

 lacral areas. Of these there are five, which alternate with the amhulacral areas in the 

 architecture of the test : the poriferous zones form the line of demarcation between these 



* Albin Gras, 'Description des Oursins Fossiles de departement de I'ls^re,' p. 20. 

 E.Desor, 'Synopsis des Echinides Fossiles,' p. 1. 



