THE ECHINODERMATA 275 



lost its jaws and teeth. From the beginning we thus 

 find two well-marked and divergent orders. 



ORDER i : GNATHOSTOMATA. Mouth central, with 

 girdle and lantern (the latter sometimes vestigial). All 

 ambs similar. Symmetry bilateral, but with strong 

 radial tendency. 



Two divergent sub-orders are found : (i) Holectypina, 

 in which the jaws are reduced in size and strength, the 

 pores of the ambs extend in unbroken series from apex 

 to base, and some of the amb-plates may be compound ; 

 and (2) Clypeastrina, in which the jaws are very powerful, 

 and the ambs tend to be petaloid, but there are no com- 

 pound plates. 



The chief genera of (i) are Pygaster (Jur.-Cret, 

 Fig. 79, a), with periproct close behind apical disc ; 

 Holectypiis (Jur.-Cret., Fig. 79, b), in which it has shifted 

 to the under side ; Discoidea (Cret.), hemispherical, peri- 

 proct marginal, with ten radiating ridges internally around 

 mouth, well seen on internal casts ; and Conulus (Cret.), 

 already described. 



In (2) the tendency is for the shape to become very flat 

 (cake-urchins) : this diminishes the strength of the corona 

 against vertical pressure, and two methods are adopted 

 to strengthen it. The more usual is for vertical pillars or 



FIG. 79. IRREGULAR ECHINOIDS. 



a, Pygaster morrisi Wright, Bathonian. Aboral view. I X J. ) a', Apical 

 disc of P. umbrella Agassiz. (X!) 6. Holectypus depressus Leske, 

 Bathonian. Oral view. (Xj.) b' ', Apical disc. (Xi.) c, Clypeui 

 altus M'Coy, Vesulian. Aboral view. (Xj. ) c'. Oral view. (Xi.) 

 c", Apical disc, -(x-J-) c'", Interamb plate and adjacent part cl 

 amb. (X2.) d, Clypeaster suffarcinatus Duncan and Sladen. Aboral 

 view, d', Oral view, d", Side view. (AllXj.) a,b,c t After Wright ; 

 d, after Duncan and Sladen. In all, except the apical discs, the 

 sutures between the plates are omitted ; in a and b, pore-pairs are 

 indicated by single dots. In the apical discs the madreporite is dotted ; 

 the ocular plates are distinguished by smaller size and absence of 

 genital pores ; in b', the left anterior ocular plate and pores on four 

 genital plates are accidentally omitted. A, periproct ; A.F., actinal 

 furrow ; Ap. , apical disc ; D, bourrelct ; M, peristome ; Ph. , phyllode 



