GENEKA AND GEOTJPS OF THE ECHINOIDEA. 267 



Ambulacra similar, apetalous, flush, with high plates ; the an- 

 terior is long ; the antero-lateral ambulacra are slightly arched, 

 concavity in front, and the posterior are wide and ornamented 

 on either side of the plastron ; number of plates small ; zones 

 increasing in width to the ambitus and then diminishing to the 

 peristome. Pores solitary. Tubercles in the ambulacra small 

 and from one to four to a plate with miliaries. 



Interradia wide, especially the anterior, with but few plates, 

 and these are high and carry several tubercles, whicli increase iu 

 number and size actinally. 



Peristome excentric in front, semilunar, with a broad, long, 

 posterior labrum. Large pedicels around the mouth. The 

 sternum is low, and with a true plastron (amphisternous) on 

 which the tubercles are slightly crowded. There is a slight pos- 

 terior actinal keel. The periproct is small, posterior, and either 

 close above the ambitus or at a little distance from it, and 

 superior to a projection. 



The spines are small, straight, cylindrical, and largest actinally ; 

 all are short and delicate. There are no fuscioles. 

 'Recent. Antarctic Ocean, from 1950 fathoms. 

 A. Agassiz remarks that the shape of the test varies, and some 

 young forms are conical; but with age the test is more de- 

 pressed and the keel becomes prominent, whilst the periproct 

 gets nearer the ambitus. 



The colour varies from violet to a dirty green. 

 The ambulacra of this genus are narrow in relation to the in- 

 terradia, and in both areas there is a remarkable paucity o£ 

 plates, and as the apes is far back the anterior ambulacrum is 

 long, and the anterior interradia are large. 



" The ambulacral pores are shown to be single in the drawing 

 given by Loven in ' Pourtalesia,' p. 76, and it is evident that 

 there is some variability iu the structure of the apical system 

 (see A. Agassiz, Eeport on ' Challenger ' Echini, pi. xxxv a). 



A. Agassiz considers that Genicopatagus has striking affinities 

 with Solaster, Cardiaster, and Toxaster. He notices that the 

 ambulacra in Genicopafagus are slightly sunken as in Toxaster, 

 that their structure is that of Cardiaster, and that the outline of 

 the test recalls that of Solaster. He shows that the genus 

 differs from Palceopneustes. He considers that the apical system 

 is more like that of Cardiaster, not being so elongate as in 



