ECHINODERMA TA S YSTEMA TIC RE VIE W 



291 



/, etc. (principally fossil), (b) Ambulacral apophyses rudimentary or want- 

 ing : Discoidea, C&noclypeus (fossil). 



Order 4. Clypeastroida. 



Mouth central or sub-central. Anus outside of the apical system in the posterior 

 interambulacrum. With external gills. With tentacle pores in the interradii. 



FIG. 232. EcMnocidaris (Arbacia) pustulosa, from the apical side (original). The spines have 

 been removed from part of the shell. 1, Interambulacrum ; 2, ambulacrum. 



More than one pair of pores on each ambulacral plate. Tentacles differ in one and 

 the same animal. Teeth usually almost horizontal, rarely vertical. The jaws lie 

 above the apophysial ring, which is interrupted. Sphseridia present. 



The test is seldom much arched ; it is usually more or less flattened, and often 

 even disc-like. It often has many incisions and perforations, and is usually bilater- 

 ally symmetrical. Its dorsal wall is connected internally with its ventral wall by 

 means of pillars, needles, septa, etc. Basal plates of the apical system fused. The 

 ambulacra form petaloids in the apical region. 



Fam. 1. Fibulariidae : Echinoeiiamus, FibvJaria, etc. (extant and fossil). Fam. 

 2. Clypeastridse : Clypeaster (Fig. 233), etc. (extant and fossil). Fam. 3. Laganidse : 

 Lagan urn, (extant and fossil). Fam. 4. Scutellidse. In all the genera of this family 



