46 ECHItfODERMATA. 



ClypeastridcB. Cassidulidce. Spatangus. 



Echinocyamus. *Cassidulus. Agassizia. 



Fibularia. Echinoneus. Breynia. 



Clypeaster. Echinolampus. Palaeostoma = Leskia. 



Laganum. *Ceratomus. Amphidetus = Echi- 



,,;,.,., *Dysaster. nocardium. 



Melhtidcs. Brissus. 



Mellita. Anochanua. Metalia. 



Rotula. Meoma. 



Encope. Spatangidce. Linthia=Desoria. 



Echinarachnius. Anaachytes. Schizaster. 



*Scutella. Pourtalesia. Moira. 



Order III. PAL^CHINOIDEA. 



TESSELLATA. 



Three to six rows of plates in each interambulacral space. 

 This order is confined to the Palaeozoic formations. There 

 but one family : 



Palachinidce. 



*Palaechinus. 



*Melonites. 



*Eoeidaris. 



Class IV. HOLOTHTTRIOIDEA. 



SCYTODERMATA. 



Body cylindrical or vermiform, with a coriaceous skin, in which 

 are scattered calcareous particles, and, with few exceptions, fur- 

 nished with five longitudinal rows of ambulacra! suckers or tube- 

 feet. Mouth tentaculate. 



The tentacles surrounding the mouth are plumose and retractile, 

 and generally from ten to twenty in number ; they are prolonga- 

 tions of the water-vascular system. The anal aperture is at the 

 opposite extremity. The intestine is convoluted and often com- 

 plicated. The skin is lined with powerful longitudinal and trans- 

 verse muscles, by which the animal is enabled so to contract its 

 body as to eject all its viscei-a. In this condition it will live for 

 some time, and in three or four months the lost parts will be re- 

 produced. Some members of this class will divide spontaneously 

 into two or more parts, each developing new and perfect organs. 



