TERMINOLOGY. 28 



Cidaridce, the mouth-opening is central, circular, or slightly pentagonal (PL IV, fig. 16) ; 

 but in Hemicidaris, Pseudodiadema, Hemipedina, Pedina, Echinus, and other Echinidce, the 

 mouth-opening is more or less decagonal, its margin being divided by notches (entailles) 

 into ten lobes ; the lobes, in general, are unequal in size, those corresponding to the 

 base of the ambulacra being the largest; they are called the ambulacra! lobes ; corre- 

 sponding to the base of the inter-ambulacra are the inter-ambulacral lobes. The margin of 

 the mouth-opening is called the peristome, to it the buccal membrane which closes the 

 base of the test is attached. 



The mouth-opening is central and armed with jaws in the Cidarid.e, Echinid^e, 

 Salenid^e, Galeritid^e, and Clypeasteridjs. It is more or less excentral and edentulous 

 in the Echinoinid^b, Collyritid^e, Echinolampid^e, Echinocoryd^e, and Sfatangid^e; 

 in them it is round, oval, or pentagonal ; sometimes its margin is ring-like, or surrounded 

 by five prominent lobes ; in others it is distinctly bilabiate. 



The vent, or anal opening, is always in the upper surface, in the centre of the genital 

 and ocular plates, directly opposite to the mouth, and is either central or subcentral in 

 the CiDARiDiE, EcmNiDiE, and Salenidje (PI. VI, fig. 1). In other families its position 

 varies much ; sometimes it opens on the upper surface, as in some Galeritidje and 

 Cassidulio.e. Sometimes it opens on the margin or is supra-marginal, marginal or 

 infra-marginal ; often it opens at the base between the mouth and the border. During the 

 life of the animal this opening is closed by an anal membrane and a series of small angular 

 anal plates ; their number and disposition varies 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 and the vent. PI. VI, fig. 1 a, is a magnificent specimen of 

 Cidaris sceptrifera, Mant, belonging to the British Museum, in which the anal plates are 

 finely preserved in situ. 



The Ambulacral and Inter-ambulacral Areas. 



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

 plaquettes, Tafelchen, Assula of authors ; they are pentagonal in form, and 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 narrowest. 2nd. Of a series of hexagonal 

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

 ten rows of small plates, notched on their margins to form holes ; these form the 

 poriferous zones. 4th. Of moveable spines, that are jointed with eminences on the outer 

 surface of the columnar plates. 



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

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

 in the test of the Echinoidea. In the Cidarid^e the ambulacral areas are very narrow, 

 and support only granules (PI. VI, fig. 1, a, b, c, d). In the EchinidjE, they are much 



