ON THE EXPERIMENTAL HYBRIDIZATION OP ECHINOIDS. 331 



Echinus acutus — • 



Diameter of test, 275 cm. Height of test, 2 cm. Age unknown. 



The amhulacral plates. A primary spine occurs only on every second plate, 

 except the thi'ee or four lowermost plates, which have each a primary spine. 

 Secondary spines are not found on the aboral side. At the ambitus the plates 

 without primary spines have a secondary spine at the outer edge and another at the 

 inner edge, the latter being the larger. These secondary spines begin about the 

 twelfth plate from above, and are i-epeated successively downwards as far towards 

 the peristome as the alternation of the primary spines. The number of amhulacral 

 plates in each series is about thirty. 



Interambulacral plates all with a primary spine. On the fifth to the sixth plate 

 from above, a pair of secondaiy spines appear at the upper edge of the plate near the 

 primary spine. These are repeated on successive plates downwards almost to the 

 peristome. On about the eighth plate from above another secondary spine appears 

 at the median corner, and this is repeated likewise nearly to the peristome. They 

 form a median series of spines in this area. Other secondary spines do not occur. 

 The number of plates in each series is about sixteen to seventeen. The longest spines 

 measure about I cm. in length. 



The apical plates are of the typical shape. All the oculars are excluded from the 

 periproct. The genital pores have begun to form. There are two to three spines at 

 the inner edge of each genital plate. There are no spines on the ocular plates. 

 The anal plates are rather large and are without spines. The suranal plate is only 

 slightly larger than the adjoining plates. 



The buccal plates are without spines ; other plates in the buccal membrane are not 

 to be discerned. 



Colour of the test on the aboral side is a light brown. The median line in both 

 amhulacral and interambulacral areas white, as are also the pore ai'eas. The apical 

 plates each have a brown spot. The anal plates have brown borders. The spines are 

 light brown at the base, the colour gradually disappearing towards the tip, which is 

 white. The oral side is white, except for a slight greenish tint at the base of the 

 spines. The tube-feet are white. 



Echinus miliaris (Psammechinus) — 



Diameter of test, 2 cm. Height of test, 1 cm. Age, one year. {Cf. Plate 25, 

 fig. 125). 



Amhulacral plates all with a primary spine. On the fifth to the sixth plate 

 coxmting from above a secondary spine appears on the upper inner edge. This is 

 repeated in successive plates downwards almost to the peristome, and forms a 

 conspicuous double series of spines along the middle hne of the ambulacra. On about 

 the ninth plate from above a secondary spine appears at the outer edge of the plate, 



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