ECHINOIDEA. 243 



Cretaceous, except Pygastrides * Loven from the Caribbean Sea, 200-300 

 fathoms. The fossil genera are Holectypus Desor, Pileus Desor, Pygaster 

 L. Ag., without the posterior basal ; Discoidea Klein ; Conoclypeus 

 L. Ag. Galeropygus Cotteau is allied here. 



Sub-Order 4. CLYPEASTROIDA (Cake-urchins). 



Mouth central or nearly so ; anus outside the apical system in the 

 posterior interradius and is either on the lower side or at the margin. 

 With external branchiae and well-developed jaws and jaw skeleton. 

 Tube-feet heteropodous. The edge of the corona is usually close to the 

 mouth and there are no perforated ambulacra! plates in the peristomial 

 membrane. Peristomial margin consists of 10 ambulacral f and usually 

 5 interambulacral plates. Teeth visually more or less horizontal, rarely 

 vertical. Sphaeridia are present, few, and covered. The test is usually 

 flat, rarely arched dorsally ; its margin may be notched. Its cavity is 

 traversed by calcareous pillars and septa which pass from its upper to its 

 lower walls. The apical system is much reduced. The madreporite is 

 central (Fig. 159) and the basal plates are coalesced with each other and 

 sometimes with the oculars. The madreporite may spread out on to all 

 the apical plates, and the generative openings may descend into the 

 interambulacral areas (p. 218). There are at least four genital openings, 

 visually five ; when only four the posterior is absent. There are five petals 

 (Fig. 152) with pairs of pores, which pass out between the plates and are 

 generally yoked. There are numerous small simple tube-feet, each 

 one in relation with one pore ; for the arrangement of these see p. 215. 

 The interambulacral plates may be interrupted (p. 217) and often have small 

 single pores, at least on the lower surface of the shell. The interambulacral 

 marginal peristomial plates are single ; the perignathic girdle is discon- 

 tinuous. In the young state the shell is regular and there are five equal 

 interradii. Cretaceous to the present day. 



Fam. 1. Fibulariidae. Small forms with rudimentary, widely open, 

 few-pored petals, pores of petals not yoked ; jaws rather high, teeth 

 superior and slanting. Perignathic processes broad, low, one on each 

 interradius. Interambulacra small, terminating in a single apical and a 

 single peristomial plate. Periproct visually on the lower surface. Slightly 

 developed vertical partitions within the test, limiting the ambulacra at their 

 side on the oral surface and radiating towards the peristome. A sphaeridium 

 in each ambulacrum, covered. Echinocyamus van Phels., f and r, madre- 

 porite with only one pore, pore-pairs not yoked. E. pusillus Gray, N.E. 

 Atlantic and Brit. Seas, Mediterranean, etc. ; Sismondia Desor, Runa A. 

 Ag. ; Fibularia Lamk. r and f ; Moulinsia L. Ag. ; Rotuloidea R. Etheridge. 



Fam. 2. Clypeastridae. Petals well developed ; visually unequal ; 

 actinal furrows straight. Small pores scattered, generally on interam- 

 bulacra as well as on ambulacra and not specially confined to furrows. 

 Interambulacra actinally discontinuous, one peristomial and two apical 

 plates in each. Sphaeridia, two in each ambulacrum, covered. Perignathic 



* Loven, Bih. Svenske Akad. Hand., 13, 1888. 



f In some Clypeastroids two small tube-feet, each with a separate pore, 

 perforate the edge of each pair of marginal ambulacral plates (J. Miiller, 

 op. tit., p. 39). These seem to be the only representatives of the marked 

 oral tube-feet of Spatangoids. 



