vui EGHINODERMATA SYSTEMATIC REVIEW 293 



the shell is very flat : Scutelfa (Fig. 234), Eckinodiscus, Eni-npf, J/<7//7 (Fig. 235), 

 Rof.ula, Arachnoides, etc. (extant and fossil). 



FIG. 235. Mellita testudinata? from the oral side (original). 



Order 5. Spatangoida. 



Mouth central, sub-central, or on the anterior edge of the oral surface of the test 

 Anus outside the apical system, in the posterior interradius. External gills, jaws. 

 teeth, and perignathous apophysial ring wanting. Sphaeridia present. The ambu- 

 lacra generally form apical petaloids. The test is bilaterally symmetrical, arched, 

 often heart-shaped. 



Sub-Order 1. Cassiduloidea. 



Fam. 1. Echinoneidse : Echlnoconus, Echinoneus, Oliyopyyus, Ecki-itolrissus, etc. 

 (extant and fossil). Fam. 2. Cassidulidse : Cassidulus, Catopyyus, Ctypeus, Pygurus, 

 Echiiiohi.tnpas, etc. (mostly fossil). Fam. 3. Collyritidse : CoUyrites, Disaster, etc. 

 (fossil). Fam. 4. Plesiospatangidse : Eolainpas, Archiacia, etc. (fossil). 



Sub-Order 2. Spatangoidea. 



Fain. 1. Anan-chytidse : Echinocorys, Holastrr, Hemiimfwstcs, Cardiaster, Urc- 

 Cystech/inus, Calymne, etc. (the last three genera extant, the rest fossil). 

 Fam. 2. Spatangidse Group 1, Adetes : Isastcr, Echinospatagus, ffeterolampas, 

 ijus, etc. (almost exclusively fossil) ; Group 2, Prymnadetes : Hemiastcr, 

 Linthia, ScMzastcr (Fig. 236), Agassizia (extant and fossil) ; Group 3, 

 Prymnodesmia : Micraster, Brixxtts, tipiiffuii'/oiiiorpha, Brissopsis, Spatangus, Palccop- 

 ir'tixfi-x (Fig. 237), EehinoeardiuHi, LnVfitin, etc. (extant and fossil) ; Group 4, 

 Apetala : Genieopatag^is, Palceobrissus, Accstc, Aerope, etc. (extant and fossil). 



