SUB-CLASS II EUECHINOIDEA—CIDAROIDA 223 
Sub-Class II. EUECHINOIDEA. Bronn. 
Test composed of two vertical rows of plates in all of the ambulacra and inter- 
ambulacra.  Periproct either within the dorso-central system or in the posterior inter- 
ambulacrum at a variable distance beyond.  Peristome actinal, rarely anterior. 
Jaws present or absent. Permian to Recent. 
The older division into the two orders of Desor, Regulares and Irregulares (= Endocyclica 
and Exocyclica, Wright), with their further division into two sub-orders each (Holostomata and 
Glyphostomata of the endocyclic, and Gnathostomata and Atelostomata of the exocyclic orders), 
is now superseded by the erection of five independent orders, as follows :—Cidaroida, Diadema- 
toida, Holectypoida, Clypeastroida, and Spatangoida. Of these the first two correspond to the 
Regulares, and the remainder to the Zrregulares.! 
Order 1. CIDAROIDA. Duncan. 
“ Huechinoidea with an actinal, central peristome and an abactinal periproct, 
situated within the dorso-central system ; with internal branchiae only, and having 
jaws and more or less vertically placed teeth, and a discontinuous perignathic girdle ; 
the interambulacral as well as the ambulacral plates continued beyond the peristome to 
the true mouth. Spheridia absent” (Duncan). 
Family 1. Cidaridae. Wright. 
Test spheroidal , the ambulacra narrow, composed of very numerous low plates, 
which are usually primaries, rarely compound, each with a pair of pores arranged in 
single or rarely in double series ; interporiferous areas with large and small granules 
only. Interambulacra broad, composed of few plates, most with a large primary 
scrobiculate tubercle, secondaries and granules. Apical system large, with five basal 
and five radial plates, each with a perforation. Permian to Recent ; maximum in 
Jura and Cretaceous. 
SECTION A. AMBULACRAL PAIRS OF PORES UNISERIAL. 
Cidaris, Klein (Figs. 356-358). dmb undulating or nearly straight, the 
pores variable in their distance, and united by a groove or not. Amb coronal 
plates five to eleven in each column. Apical system large. Primary spines 
very variable, even in the same species. Permian to Recent ; chiefly Jurassic 
and Cretaceous. 
Of this genus more than 200 species have been described. These are grouped into seven or 
eight artificial divisions, which are regarded by some as of sub-generic, or even generic import- 
ance. The groups may be briefly noticed as follows :— 

1 [The progress of the zoology and morphology of the recent Hchinoidea during the last couple 
of decades has been such as to necessitate numerous changes in the older terminology and classifi- 
cation. The framing of an improved system, based upon the new grounds and expressive of the 
modern views entertained in regard to taxonomic relationships, was the last undertaking of the late 
Professor P. Martin Duncan. His essay on the “ Revision of the Genera and Great Groups of the 
Kchinoidea,” published by the Linnean Society, has been of the greatest service in extending the 
descriptions of numerous fossil genera in the present chapter. Following the example of Lang and 
others, Professor Duncan’s classification has been adopted almost entirely. A word of special 
acknowledgment is also due to Mr. W. Percy Sladen, for his advice and generous assistance in 
editing this portion of the Hchinodermata.—TRans. ] 
