318 THE ECHINOIDEA 



support than it could obtain from the margin of the test ; this is given by 

 numerous pillars which connect the upper and lower walls. The external 

 margin is often notched, as in Scutella ; the notches may deepen into 

 " slits " separated by finger-shaped processes, as in Rotula (Fig. IX.). In 

 some genera two adjacent processes unite at their free ends, and a hole 

 is left through the test ; such holes are known as " luuules," and occur in 

 Mellita, Monophora, etc. All the interambulacral areas are discontinuous 

 in some genera, e.g. Encope, but in Rotula and Mellita one or two of the 

 areas may be continuous from peristome to apex. 



FAMILY 4. CLYPEASTRIDAE. Clypeastrina with closed petaloid 

 ambulacra. The interambulacral plates are discontinuous ; the peri- 

 stomial plate is small. There are two perignathic processes in each 

 area, and they are ambulacral in position. The actinal furrows arc 

 straight. SUB -FAMILY 1. CLYPEASTRINAE. Massive Clypeastridae with 

 closed petals; usually high. Genera Clypeaster, Lam.; Echinanthus, 

 Leske ( = Diplothecanlhus, Dune.) ; Plesianthus, Dune. ; Anomalanthus, 

 Bell; Monostychia, Laube. SUB-FAMILY 2. ARACHNOIDINAE. Flat, low 

 Clypeastridae with open petals. Genera Arachnoides, Ag. ; Alexandria, 

 Pfeffer. These genera are usually included as a sub- family of 

 Scutellidae, which they resemble in external form. Their structure, 

 however, allies them more nearly with the Clypeasters, with which they 

 agree in all fundamental characters. They differ from the Scutellidae 

 by having (1) a very small peristomial interambulacral plate, which in 

 some species may be absent in several areas ; (2) straight, simple, actinal 

 furrows ; (3) five pairs of ambulacral perignathic processes. 



ORDER 2. Atelostomata, Zittel. 



Echinoidea Irregularia, in which there are no jaws, teeth, perignathic 

 girdle, or external branchiae. 



The Atelostomata introduce three additional structures, upon which 

 the classification within the order depends. These are the sternum, 

 floscelle, and fasciole. In Echinoids previously considered the mouth is 

 central or sub-central, and the five areas around it are of equal import- 

 ance ; but as the mouth becomes eccentric in position, one interradius 

 necessarily becomes longer than the rest. The anus is situated in this 

 interradius, which requires some modification of the plates for the sake of 

 increased strength. In the simplest of the Atelostomata the plates of the 

 posterior interradius are but slightly different from those of the other areas 

 but the plates are larger, and dovetail more deeply into one another. 

 In Collyrites there is a slight advance on this plan, and in genera such as 

 Echinocorys and Holaster the plates dovetail so deeply as to form a strong 

 sternum along the under side of the test. This type is known as the 

 " meridosternous " (Fig. XXXVIII. 1). In the next stage the first pair of 

 plates in the interambulacrum increase in length, and both are in contact 

 with the peristomial plate of the same area, as in Toxaster (Fig. XXXVIII. 2). 

 This is the " amphisternous " type, the extreme form of which we see in 

 Spatangns purpureus (Fig. XXXVIII. 4). 



