378 ECHINOZOA. 



The order of the Euechinoids includes all the normal Sea- 

 urchins, and the distribution of the group in time is remarkable. 

 No Euechinoid has hitherto been detected in any Palaeozoic de- 

 posit anterior to the Permian, but a vast development of the forms 

 of this division takes place in rocks of Mesozoic age, while all the 

 Tertiary and Recent Urchins also belong to this group. In the 

 Jurassic and Cretaceous rocks the Euechinoids attain their maxi- 

 mum development, and very numerous forms are also known from 

 the Tertiary formations. 



The Euechinoidea may be divided into the two sub-orders of the 

 Regulares and Irregulares, the characters and chief groups of which 

 will be briefly treated of in the following pages. 



Regular Euechinoids. 



The sub-order Regulares (Echinoidea endocyclica) includes all those 

 Euechinoids in which the test is "regular," the mouth being inferior 

 and central, while the anus occupies the centre of the superior sur- 

 face, and is surrounded by the apical disc. The form of the test is 

 usually spheroidal, and the ambulacral areas are perfect, and are all 

 alike. The mouth is furnished with a masticatory apparatus. In 

 Tetracidaris the interambulacra are in part four-rowed, but in all 

 others they are two-rowed, as are the ambulacra in all cases. The 

 members of this sub-order appear to be the most ancient forms 

 of the Euechinoids, being tolerably well represented in the Trias, 

 and even occurring in the still older Permian formation (Eocidaris.) 

 They are largely represented in the Jurassic and Cretaceous forma- 

 tions, but begin to diminish in numbers 

 in the Eocene Tertiary. The Regulares 

 may be divided into the four princi- 

 pal families of the Cidaridce, Salenidce, 

 E c kino thur idee, and Glyphostomata, the 

 more important forms of which are 

 noticed below. 



Family i. Cidaridce. — In this family 

 the test is spheroidal, and more or less 

 flattened at the oral and anal poles. 

 The ambulacral areas (fig. 249), are 

 Fig. 249.— Under surface of the test very narrow, often flexuous, and never 

 ^ C ^k!^?W^S!) nata,Bl Provided with large tubercles. The 



interambulacral areas are wide, and 

 carry large perforated tubercles, surrounded by an areola, and 

 supporting the primary spines. The spines are of two sizes, the 

 primary ones usually more or less cylindrical, clavate, or fusiform, 

 and generally longitudinally ridged or tuberculate (fig. 242). 



