390 



ECHIN0Z0A. 



and, as in the preceding two families, teeth are invariably wanting. 

 The tubercles of the test are mostly small, and carry hair-like 

 spines ; but there are larger, crenulated, and perforated tubercles for 

 the support of larger spines. As a rule, bands of microscopic tuber- 

 cles known as "fascioles" (fig. 268) are present, and occupy differ- 

 ent positions in different genera. 

 Sometimes the " fasciole " sur- 

 rounds the ambulacral rosette, 

 when it is said to be "peripeta- 

 lous " ; sometimes it is " internal," 

 surrounding the unpaired ambu- 

 lacrum ; sometimes it surrounds 

 the sides, and is said to be 

 " lateral " ; at other times it runs 

 round the test, and is termed 

 " marginal " ; and, lastly, it may 

 be limited to the base of the anal aperture, when it is termed 

 " sub-anal." 



Fig. 268. — Gualteria Orbignyana, viewed 

 both from above and below. The left-hand 

 figure shows the "fasciole" cutting the ambu- 

 lacral rosette. Eocene. 



Of the genera of Spatangidce characterised by a pentagonal mouth, the 

 chief is Toxaster, the species of which are wholly Cretaceous. Of the 

 more normal Heart-urchins, with a bilabiate mouth, Micraster (fig. 267) 

 possesses a sub-anal fasciole, and is widely distributed in the Chalk, the 

 genus being also represented in the Tertiary rocks of Australia. Epiaster, 

 likewise found in the Chalk, has no sub-anal fasciole, but is otherwise 

 similar to Micraster. Hemiaster, very abundant in the Cretaceous 

 period, but also represented by Tertiary and living species, has a peri- 

 petalous fasciole. Linthia (Periaster) has both a peripetalous and a 

 lateral fasciole, and ranges from the Chalk to the present day. Gualteria 

 and Macropneustes are Eocene types ; while Sckizaster, Brissiis, Bris- 

 sopsis, Echinospatagus {Amphidetus), Euftatagus, and Sftatangits are 

 well-known Tertiary and Recent genera. 



