THE ECHINOIDEA 



317 



they are narrower than the ambulacral plates in the extra-petaloid portions 

 which expand laterally, and are much broader than the interambulacra. 

 In the simplest members of this group the interambulacra are, however, 

 " continuous " from apical system to peristome (as in Laganum) ; but in 

 the more advanced, such as Rotula, the interambulacra are " discontinuous," 

 the ambulacra meeting one another and cutting off the interambulacral 

 peristomial plate from its connection with the rest of the interambulacrum. 

 Another feature peculiar to this sub-order is the presence of a series of 

 furrows on the lower surface of the test ; these are known as the actinal 

 furrows, and they are either straight, as in the Clypeastridae, or bifurcating, 

 as in the Scutellidae. 



FAMILY 1. FIBULAUIIDAE. Clypeastrina with ambulacra in rudi- 

 mentary open petals. The interambulacral plates are continuous. The 

 pillars are slightly developed. The perignathic girdle consists of five 



FIG. XXXVI. 



Branchial podia from peta- 

 loid portion of ambulacrum of 

 Clypeaster. 



1 2 



FIG. XXXVII. 



Eclt i nofyninii* pnsillnft. 1 , upper 

 surface showing simple petals and 

 four genital pores ; 2, the lower 

 surface showing the anus inter- 

 mediate between the central 

 mouth and the posterior border. 



single interradial processes. The peristomial interradial plate ia large. 

 Genera Echinocyamus, Phelsum (Fig. XXXVII.) ; Scutellina t Ag. ; Sis- 

 mondia, Desor; Fibularia, Lam.; Tkegaster, Pome). 



FAMILY 2. LAGANIDAE. Clypeastrina with ambulacra petaloid ; 

 numerous pores for prehensile podia occur in addition to the large pores 

 for the respiratory podia. The interambulacral plates are " continuous " ; 

 the peristomial plate is medium in size and bears a single perignathic 

 process. The actinal furrows are simple and straight. Genus Laganum, 

 Blainv. 



FAMILY 3. SCUTELLIDAE. Flat Clypeastrina with closed petaloid 

 ambulacra. The interambulacral plates are " discontinuous " in some or 

 all of the areas ; the peristomial plate is large and bears a single peri- 

 gnathic process. The actinal furrows are bifurcating. Genera Scutella, 

 Lam. (Fig. XXXV. 4) ; Echinarachnius, Leske ; Echinodiscus, Ag. ; Encope, 

 Ag.; Monophora, Ag. ; Mellita, Ag. ; Melitella, Dune. ; Astriclypeus, Verrill ; 

 Lenita, Desor ; Mortonia, Desor ; Rotula, Ag. (Fig. IX.) ; Rotuloidea, 

 Etheridge ; Moulinsia, Ag. ; (?) Runa, Ag. The most striking feature in 

 this family is the extreme thinness and flatness of the teats. In some 

 species, such as Scutella striatula, the test may be 100 mm. in diameter, 

 and only 10 mm. in height. The upper surface accordingly needs greater 



