332 MESSRS. C. SHEARER, W. DE MORGAN, AND H. M. FUCHS 



likewise repeated on successive plates down nearly to the peristome. The number of 

 ambulacral plates in each series is about eighteen. 



Interambulacral flates all with a primary spine. On the second to third plate 

 from above a secondary spine appears at the upper inner edge. On the third to 

 fourth plate another secondary spine appears at the outer upper edge, and on the 

 subsequent plate one at the outer edge and one at the median corner of the plate. 

 All these spines are repeated downwards almost to the peristome. Those at the 

 outer and inner side of the plate are the largest, whilst the two first to appear 

 nearest the primary spine remain small. All these spines together form a rather 

 dense covering, in which, however, the primary spines distinctly remain the longest. 

 The length of the primary spines is about 6 to 7 mm. 



The apical plates are of typical form. The ocular plates are all excluded from 

 the periproct. The genital plates each carry one or two spines at the upper edge. 

 The ocular plates each one spine in the middle of the plate. 



Genital openings distinct. The anal plates are not very conspicuous. The suranal 

 plate is slightly larger than the adjoining plates. No spines on the anal plates. 



The buccal membrane is covered with large plates arranged close together. No 

 spines on the buccal plates. 



Colour of the test uniformly green, only the anal area is whitish. The oral side 

 of the test is almost white. The spines are green, the primary ones pink at the tips. 

 On the oral side the spines are more or less distinctly banded with green and white. 

 Tube-feet white. 



E. esculentus ^ X E, acutus $ (Cf. Plate 24, figs. 100 and 104, and Plate 25, 

 figs. 123—126)— 



A number of these hybrids have survived and have attained considerable size by 

 the end of the first year. One example two years old measures more than 8 cm. 

 across the spines. 



The one from which the following description has been taken is one of the smaller 

 individuals from last year's crosses. It has been selected on account of its size, which 

 approaches more that of the other hybrids. This cross has always grown more 

 rapidly than any of the others, despite the fact that laboratory conditions must be 

 unfavourable to it on account of the deep-water habitat of both parent forms. 



With regard to this individual urchin the following are the data pertaining to the 

 culture from which it was derived :— 



