PLATK XI. 27 



of the same species as exhibited in the forms of those 

 represented in pi. x, figs. 2 — 7, accompanying the one 

 (fig. 1) on which the author bases the characters of 

 his proposed, new genus. 



Independent of the well-known and universally - 

 acknowledged protean habits of the spongiadge, which 

 renders form one of the least valuable characters either 

 specific or generic, it is in the genus Polymastia more 

 than usually inappropriate. Thus, in P. ornata, bul- 

 bosa, and i^adiosa, we have hitherto known but one 

 fistular projection forming the sponge ; and even in 

 P. mammellaris it is not an uncommon circumstance to 

 find young specimens with but one cloacal projection, 

 as represented in pi. xi, fig. 2, while in mature ones 

 they are much too numerous to be readily counted. 

 The mammasform projection on the summit in Mr. 

 Norman's specimen is evidently an abnormal structure, 

 as it is not to be found in any other specimen of the 

 species with which I am acquainted ; it is, therefore, 

 perfectly inadmissible in a generic description. 



Polymastia spinuia, Bowerbank. 



Vol. ii, p. 66, ' Mon. Brit. Spongiadsc.' 



Fig. 10. — A fully-developed specimen of the species 

 with six fistulas dredged in from forty to fifty fathoms 

 from five to seven miles off" Balta, by the Rev. A. M. 

 JSTorman. Natui-al size. 



Fig. 11. — A skeleton s]Diculum from one of the 

 primary, longitudinal fasciculi. X 123 linear. 



Fig. 12. — One of the spinulate, external, defensive 

 spicula. X 123 linear. 



Fig. 13. — A specimen of P. sjnnula, from the Moray 

 Frith, presented to me by the Rev. Walter Gregor, 

 described in p. 67, ' Mon. Brit. Spongiadas.' Natural 

 size. 



Since the figures described above were drawn I have 

 received from my friend the Rev. A. M. Noi-nian a 



