2 Field Museum of Natural History — Geology, Vol. IV. 



The terminology of Desor* with some modifications is adhered to. 

 It may be briefly stated as follows : 



Echinoids or Sea Urchins: Marine animals without arms or stem, 

 the bodies encased in a solid or slightly flexible test or shell, varying 

 in shape from spherical to flat ; composed of numerous closely placed 

 plates, covered with spines. The mouth is on the ventral or actinal 

 surface; the anus either in the apical system or somewhere in the 

 posterior interambulacral area. The plates of the ambulacral areas 

 are more or less extensively perforated for the protrusion of the am- 

 bulacra or tube feet. 



Dorsal Surface: (Af) The upper, usually convex surface, on 

 which are situated the apical system, petals, etc. 



Ventral or Actinal Surface: (C) The lower surface on which the 

 peristome is situated. 



Anterior Sulcus: (as) A more or less distinct groove in the an- 

 terior margin of the test. In it is situated the anterior ambulacral 

 area. 



iamb 



iamb 



Fig. i. Diagram of Linthia variabtlis. A, dorsal view. B, posterior view. C, ventral view. 



Ambulacral (amb.) and Interambulacral Areas: (iamb.) The test 

 of practically all echinoids is divided into ten segments or areas, 

 five ambulacral and five interambulcral areas. In living echinoids 

 and most Mesozoic or Cenozoic fossil echinoids, including all species 

 mentioned in this paper, each area consists of two rows of plates. 



Poriferous Zones: On each side of the ambulacral areas are bands 

 or zones containing openings or pores through which issue the tentacles 

 or tube feet. There are ten poriferous zones, two to each ambulacral 

 area. In some echinoids there is quite a space between the poriferous 

 zones and this space is called the inter poriferous zone. 



Simple Ambulacra: Those in which the pores of the poriferous 

 zones have the same disposition from the apex to the peristome. 

 This is the case in all "regular echinoids." 



* E. Desor 1858. Synopsis des Echinides Fossiles Paris, 

 t The letters in parentheses refer to those of Figs. 1 and 2. 



