TETHTA. 39 



Genus 12. TETHYA, Lamarck, i, 181 ; n, 6. 



1. TETHYA CKANIUM, Lamarck, n, 83 ; i, PI. IV, figs. 77 



-82; PI. XXV, fig. 343; PI. XXXI, fig. 362; 



in, PI. XIV, figs. 16; PL LXXXIX, figs. 58. 



1870 Tetilla cranium, Schmidt. Spong. Atlant., Geb., p. 76. 



1872 Tethya Zetlandica, Cai-ter. Ann. Nat. Hist., ser. 4, 



vol. ix, p. 417, pi. xxii, figs. 16, 1317. 

 Tethya cranium, Carter. Loc. cit., pi. xxii, figs. 9 12. 



1875 Tethya cranium, vars. dbyssorum and infrequens, Carter. 

 Ann. Nat. Hist., ser. 4, vol. xviii, p. 405, pi. xvi, fig. 49 

 (abyssoruni), and fig. 48 (infrequens). 



Habitat. Dredged in deep water in the Minch in 

 company with Phakellia ventilabrum, Isodictya infun- 

 dibuliformis and laciniosa, and Normania crassa, which 

 are also its companions in the deep waters of the 

 Shetland Seas (N.). 



Either there must be an extraordinary amount of 

 variation in the character of the spicula of this sponge, 

 or there are in the seas around the British Islands very 

 closely allied species which, while agreeing in general 

 character, may be distinguished by certain variations in 

 their spicula. It is not without much hesitation that I 

 here regard these forms as variations of a single species. 

 So regarded, I cannot but feel that they give a very 

 serious warning against basing specific character on 

 minute spicular differentiation ; and in these volumes a 

 large number of species are established on the ground 

 of minute spicular variation. At the same time it must 

 be carefully borne in mind that in all other classes of 

 animals the amount of plasticity of form and structure 

 varies immensely, even in kindred species ; and the fact 

 that one animal or plant varies almost endlessly does 



