24 ECHINOIDEA. 



wider, and have large tubercles on their surface. The comparative width of the ambulacra 

 as compared with the inter-ambulacra has led some authors l to divide the family Cidarida, 

 including therein the Echinidce, into two tribes, the Angustisteel.*:, or Cidaridae with 

 narrow ambulacra, and the LatistelLuE, or Cidaridoe with broad ambulacra. These two 

 tribes nearly represent our two families ; the Cidaridoe are equal to the Angusti steels, 

 and the Echinidje are nearly equal to the Latisteel.e. 



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

 spheroidal Echinidce and Salenida. This is detected by its relation to the apical disc, 

 as the right antero-lateral plate always carries the madreporiform body ; in the oval, 

 pentagonal, or elongated forms, its position and relation to the mouth renders it unmis- 

 takeable. The four other ambulacra are disposed in pairs. 



The inter-ambulacral plates form two broad columns, composing the inter-ambulacral 

 areas ; of these, like the ambulacral, there are five, which alternate with them in the archi- 

 tecture of the test. The poriferous zones form the line of demarcation between these two 

 classes of columnar plates. The plates are all pentagonal, and many times larger than 

 the ambulacral ; they carry on their surface the large primary tubercles. Of the five iuter- 

 ambulacral areas, one is single and posterior, and in all the Echinoideas which have the anal 

 opening external to the apical disc it is in the single inter-ambulacrum that the vent ter- 

 minates. The other four inter-ambulacra are disposed in pairs, and form the greater 

 part of the anterior and lateral parts of the test ; they are the anterior pairs and posterior 

 pairs, respectively, to distinguish them from the odd area, which is the single inter- 

 ambulacrum. 



PI. VI, fig. 1, shows the form and structure of the inter-ambulacra in the Cidarida. 

 In the Cidarida the inter-ambulacral areas have only two rows of primary tubercles ; 

 but in many of the Echinida there arc four, six, eight, or ten rows of primary tubercles 

 in these areas. 



The Poriferous Zones. 



Hie poriferous zones are situated on each side of the ambulacral areas (PI. VI, fig. 1) ; 

 they are composed of a very great number of small pieces, articulated together in such a 

 manner as to form a series of holes. The corresponding edges of the plates remaining 

 uncalcified, at certain definite intervals produce foramina, which are destined for the 

 passage of retractile tubular suckers. The form and structure of the poriferous zones form 

 a good generic character. As there are two poriferous zones bordering each ambulacral 

 area, it follows that there are ten zones. Some authors give the collective name ambulacra 

 to the zones and the area ; but for obvious reasons, I consider them distinct sections of 

 the test, and treat them as such. 



1 Albiu Gras, 'Description des Oursins Fossilcs du departemeut de l'lsere,' p. 20. E. Dcsor, 'Synopsis 

 des Eclmrides FossiJes,' p. 26. 



