﻿ON THE ECHINOIDEA. 



403 



ECHINOCONIDiE. 



Ptgastke semisulcatus, P/tillips. PI. XLIII, fig. 6, Supplement to pages 275-78. 



Professor Phillips found in the Inferior Oolite of Whitwell the original specimen of 

 P (/(/aster semisulcatus ; from the outline of the vent of that figure, it is still, however, 

 doubtful whether the urchin he figured in PI. Ill, fig. 17, of his ' Geology of Yorkshire,' 

 was a Whitwell specimen. I am of opinion that it was Pygaster umbrella, Ag., from the 

 Coralline Oolite of Ayton or Hildenley that formed the type, and not a Whitwell specimen 

 at all. At that time, and for long afterwards, both urchins were considered to belong to 

 one species, and it is very probable that, as much finer specimens of Pygaster were 

 collected from the Coralline Oolite than had been obtained from Whitwell, a specimen 

 from the Coralline Oolite was preferred for the drawing. I have shown in my articles 

 on these two species how perfectly distinct Pygaster semisulcatus is from Pygaster umbrella, 

 and any one carefully comparing our description with Professor Phillips's figure will at 

 once discover that the vent-opening in his drawing is the keyhole-shaped vent of 

 P. umbrella, and not the wide opening of P. semisulcatus. 



Having lately found a very good specimen of the true Pygaster semisulcatus from the 

 Inferior Oolite of Whitwell in the collection of my friend, C. W. Strickland, Esq., of 

 Hildenley, I have figured a portion of the posterior view of this urchin, with the view 

 to exhibit the form of the vent-opening. It will be observed that this aperture is much 

 smaller than the vent-opening of P. semisulcatus from the Inferior Oolite of Glouces- 

 tershire (PI. XIX, fig. 1), and does not extend so far down the single inter-ambulacrum 

 as in that specimen ; the tubercles are likewise more sparse upon the Yorkshire urchin, 

 and the mouth-opening is relatively smaller. It is important to note these characters, 

 as they belong more to varieties of a given type than to a new specific form, and serve 

 to teach us that, before the history of a species can be fully written, it is necessary to 

 collect different individuals of the same species from localities widely apart, in order that 

 we may estimate the degree of variation which changes of physical conditions were 

 capable of exercising on the secondary characters of specific forms. 



Pygaster macrostoma, Wright. Supplement, PI. XLI, fig. 4 a, b, c ; fig. 5 a, b. 



Test depressed, pentagonal ; anal opening large, wide, occupying nearly two thirds 

 of the single inter-ambulacrum; sides tumid, base convex from the peristome to the 

 border, mouth-opening large, one fourth the diameter of the test. 



