3 8o 



ECHINOZOA. 



the anus is placed in the middle line ; and the Goniophorus of the Chalk 

 is a closely allied form. Acrosalenia^ again, ranges from the Lias to the 

 Chalk. 



Family 3. Echinothuridce. — In this small but highly remarkable 

 division of the Echinoidea the test is "regular," the anus being 



Fig. 251. — Salenia fiersonata. The left-hand figure represents the upper surface of the shell, 

 and shows the anus surrounded by the apical disc. The right-hand figure shows the under side 

 with the peristomial space. 



placed in the centre of the apical disc, and the ambulacral areas 

 being continuous ; but the plates of both the ambulacral and inter- 

 ambulacral areas are imbricated and overlap one another (fig. 252), 

 the test thus becoming flexible. In this abnormal character, the 

 Echinothuridce agree with some of the Palaeozoic Urchins, but they 



Fig. 252. — Portion of one of the ambulacral areas of EchinothuriaJZoris, enlarged four 

 times. Chalk. (After Wright.) 



differ from these, and agree with the ordinary Regular Echinoids in 

 having the test composed of no more than twenty rows of plates. 



The only fossil forms of this group, as yet discovered, are refer- 

 able to the Cretaceous genus Echinothuria^ the true affinities of 

 which have now been elucidated by the discovery of the extraor- 

 dinary living types referred to the genera Asthenosoma and Phor- 

 mosoma. 



Family 4. Glyphostomata. — In this family the ambulacral areas 

 are not much narrower than the interambulacral, and both as a rule 

 carry primary tubercles. The peristomial membrane usually carries 

 irregular calcareous ossicles, but is not completely plated, and the 

 peristome is furnished with more or less marked incisions at its 

 angles. " Auriculae " are present. This family is divisible into the 



