AND ITS ALLIES. 219 



•different positions with the varying tension of the skin ; and this 

 is also Weber's view (18). 



Ehrenberg's description is very indefinite ; but if the animal 

 usually identified as P. aculeata is really that species, it is so 

 variable that all the laxity of the original diagnosis is necessary 

 in order to embrace all the forms. 



Failing the rediscovery of Ehrenberg's type, the form having 

 the largest number of spines (13, in Janson's var. medio-acideata) 

 might be regarded as the type from which all the varieties have 

 been derived by the suppression of one or more pairs. 



Weber (18) regarded as the type a form having eleven spines, 

 including a median ; but without the forward -pointing spines. 



Eckstein (5), in 1884, found always ten spines, in pairs, 

 all directed backward. 



Milne (15), 1886, found always eight spines, in pairs, one pair 

 directed forward. 



Gosse (11), 1886, describes and figures eleven spines, including 

 anterior-median and lateral forward-pointing spines, and differing 

 from Janson's medio-aculeata only by the absence of one dorsal 

 pair. 



Rousselet has mounted, and kindly lent me, examples of the 

 variety medio -aculeata (figured in PI. 17, Figs. 16-17). 



There follow short descriptions of all the forms which I have 



personally examined. Though no one, so far as I can find, has 



noticed a twelve-spined variety, that is by far the commonest 



in Scotland. 



13 Spines. 



P. acideata type = var. medio-aculeata Janson (13) (Figs. 16, 17, 

 drawn from mount by Rousselet). 



Spines : 1st row, anterior, of three spines of moderate length ; 

 2nd row, a pair of very long spines, springing from skinfolds 

 nearer the middle line than the outer spines of the 1st row; 

 3rd row, a pair of short spines, on the lateral skinfolds, directed 

 forward ; 4th row, a pair of long spines, on skinfolds farther 

 from the median line than those of the 2nd row ; 5th row, 

 a pair of short spines, on folds near the middle line ; 6th row, 

 a pair of short, divergent spines, on the segment after the 



