1909.] AN" ABNORMAL ECHINOID. 715 



others, the anterior and left-posterior, being rounded off a little 

 above the" ambitus. The apical system is elongated in two 

 directions as thovigh to meet the defaulting ambulacra, but in 

 each case a considerable length of non-poriferous plates intei-venes 

 between the adapical extremities of the two ambulacra and the 

 plates from which they should spring. 



The other three ambulacra show no departure from the normal 

 type, except that the left-anterior (IV) is sensibly wider than 

 either of the others, and less convex in longitudinal outline. 



The right posterior interambulacrum (1) is the only one of 

 normal character throughout, the other four being more or less 

 modified adapically to counteract the absence of the ambulacra. 

 The adoral region of the test is perfectly normal, and the peri- 

 stomial aperture regularly decagonal. 



Text-fis. 227. 



I 

 5 



Adapical view of test of an abnormal Amhlypneustes. 



The apical system (text-fig. 228, p. 716), besides the lengthening 

 already referred to, shows marked departures from the normal type. 

 There are five genital plates, the inner margins of which enclose the 

 elongated periproct, but they are of very unequal dimensions. 

 The madreporite is large, and perfoi-ated on the right side by a 

 small genital pore. The two adjoining genital plates on the 

 right and left sides a,re elongated and depressed. The plate on 

 the right is normally perf oi-ate, but that on the left bears two large 

 gonopores, one near each end. The two posterior genital plates 

 are short and convex, and closely applied to each other, forming 

 an elevation that matches the anterior prominence of the madre- 

 porite. The right posterior plate is perforated in the usual way, 



