M'SCRIPTIOXS OF Till: SI'KCll'.s. 247 



hundred outer side plates. Dr. C. A. "White gives a good description of them, which 



is well illustrated by our figures (PI. VII. figs. 10, 12; PI. X. tig. I 1 ; I'l. XI. fig. 1 1). 

 The hydrospire-plate, which is well seen in the last two figures, is not so steeply 

 inclined as usual, and so takes more share in supporting the; side plates. The 

 pinnules have been described by Meek and YVorthen ' as slender, simple, of uniform 

 size, and each composed of a single row of pieces, thereby not differing essentially 

 from these of /'< ntn miffs. Through the kindness of Mr. Wachsmuth we are enabled 

 to figure a specimen of G. Norwood!, with the column attached. As already described 

 by Meek and Woithen -, the joints are round and thin near the calyx, but become 

 thicker and alternately larger and smaller towards the lower part of the stem. 

 Although Mr. Wachsmuth's collection contains several good examples of this species 

 with the summit plates preserved (PI. VII. figs. 4, 11, 13), yet such cases do not 

 appear to be very common. They enable us to confirm Dr. White's description of the 

 parts in question, but we cannot agree with many of his deductions, as we have 

 explained on pp. 77, 78. The so-called " vault " takes the form of a small dome, and 

 is composed of a limited number of irregular plates of variable size and disposition. 

 The figures given by Meek and Worthen 3 exactly represent the plates as we have 

 seen them ; but we have not found the anus ever covered by a single plate, as they 

 represent it 4 . "We give a very interesting figure (PI. VII. fig. 12) of a specimen in 

 Mr. Wachsmuth's collection, in which one of the deltoids is perforated by two 

 .spiracles. This appears to lead up to the structure of the spiracles in Heteroblastus 

 (PI. VI. figs.- 3, 4), to which we shall refer hereafter. 



We have had the advantage of examining a large number of specimens of 

 G. Xorwoodi, and have succeeded in distinguishing at least six directions in which 

 variation occurs (PI. II. figs. 32-35): — 



1st. In the form of the calyx. Var. A, which we believe to be the species proper, 

 presents a simple globose outline. Var. B is elliptical-globose in outline, being 

 elongated in the direction of the vertical axis, and having a less transverse diameter, 

 in proportion to its height, than Var. A. 



2nd. In the breadth of the interradial areas. Var. A has these portions of the 

 calyx almost one third wider than in Var. B, and is therefore usually more penta- 

 gonal in outline. 



3rd. In the projection of the ambulacra. Extreme forms of the Var. A have a 

 marked inflation in the line of the ambulacra. This is so considerable that were 

 such individuals found alone, without any connecting links with other varieties, they 

 would certainly be described as new species. 



4th. In the undue projection of one or sometimes two of the ambulacra only, 

 usually on the anal side, so as to give the calyx a very unsymmetrical appearance. 



1 Proc. Acad. Xat. Sci. Philad. I860, p. 258. -' 77,/,/. p. 258. 



3 Eeport Gcol. Survey Illinois, 1873, vol. v. pi. 9. f. 2 a. ■' Ibid. pi. 9. f. 2 h. 



