316 THE ECH1NOIDEA 



This sub-order is difficult to characterise, as it includes the primitive 

 Irregular Echinoids, as well as one series of these forms which has 

 continued to the present day. 



FAMILY 1. PYGASTERIDAE. Holectypina in which the peristome is 

 large, and the perignathic girdle consists of disconnected processes. 

 The ambulacra are simple and apetaloid. Genera Pygaster, Ag. 

 (Fig. XXXV. 1) ; Pileus, Desor ; Pygastrides, Loven ; Holectypus, Desor ; 

 (?) Pachyclypeus, Desor; Galeropygus, Cott. FAMILY 2. DISCOIDIIDAE. 

 Holectypina in which the peristome is small and the perignatliic girdle 

 tubular. Jaws unknown. Ambulacra apetaloid. Genera Discoidea, 

 Gray ; Protocyamus, 1 nom. nov. FAMILY 3. GALERITIDAE. Holectypina 

 in which the perignathic girdle is rudimentary, jaws are absent, and 

 their place taken by ten buccal plates. Genera Galerites, Lamk. 

 (Fig. XXXV. 3) ; Lanieria, Dune. ; Adelopneustes, Gauth. ; (?) Copto- 

 discus, Cott. & Gauth. FAMILY 4. CONOCLYPEIDAE. Holectypina in 

 which the peristome is small ; the perignathic girdle tubular and high, 

 surrounded by five bourrelets. Genera Conoclypeus, Ag. ; Oviclypeus, 

 Dames. 



SUB-ORDER 2. CLYPEASTRINA. 



Gnathostomata in which the jaws are powerful. The teeth are 

 placed in pyramids, which articulate by a socket fitting on to vertical 

 processes ; the jaws only move horizontally, and have neither braces nor 

 compasses (p. 289). The ambulacra are petaloid. 



This sub -order includes a series of striking variations from the 

 ordinary Echinoid type. Echinocyamus is the most primitive form, and 

 appears to have developed from an ancestor closely allied to Protocyamus. 

 The great advance in Echinocyamus is the expansion of parts of the 

 ambulacra into rudimentary petals (Fig. XXXVI.) ; in the upper part 

 of the ambulacra the outer pores of the pore-pairs have increased to small 

 slits, and occur along curved lines, enclosing somewhat leaf-shaped areas. 

 Beyond these only a single pore occurs in each ambulacral plate. The 

 perignathic girdle of Echinocyamiis consists of five vertical pegs, rising 

 from the interradial peristomial plates ; this reminds us of Galerites, in 

 which the perignathic girdle is reduced to five interradial thickenings. 

 The structural differences between Protocyamus and Echinocyamus are 

 small, and their importance is exaggerated by the different shape of the tests. 

 But Echinocyamus was succeeded by a very divergent series. Most of the 

 members of the group are long, broad, and low ; some are thin and flat. 

 In these cases the upper surface regains the support it loses owing to 

 departure from the dome-shaped form, by the development of pillars which 

 pass from floor to roof. The ambulacra in the typical forms are petaloid, 

 and the podia in these areas expand to act as branchiae (Fig. XXXVI.). In 

 some cases pores only occur in the petals ; in others they are scattered over 

 the test, occurring on both radial and interradial plates. In some genera, 

 such as Laganum, though the petaloid portions of the ambulacra are broad, 



1 A name suggested in lieu of Echinites proposed by Duncan, but preoccupied by 

 . Leske for Echinoids, and by Miiller and Troschel for Asteroidea ; the name is selected 

 to indicate the affinity of this echinoid with the Echinocyamus series. 



