40 BEITISH SPONGIAD.E. 



not enable us to predicate that a closely allied form 

 will prove similarly unstable in its characters ; on the 

 contrary, it may prove exceptionally constant to its 

 type. 



A very important question still remains unanswered 

 among the sponges, viz. What amount of variation in 

 the form and proportionate size of spicula may be re- 

 garded as consistent with specific character ? No doubt 

 the amount of such variation will, as I have suggested, 

 be found to differ greatly in different genera and 

 species. Future observations must determine whether 

 the divergencies here considered as varietal in Tethya 

 cranium should or should not be rather reckoned as 

 specific. It will be easy in such latter case to employ 

 the names here used to designate the several varieties 

 as indicative instead of species. 



Dr. Bowerbank first pointed out that the numerous 

 specimens of Tethya cranium which he had examined 

 contained two sorts of gernmules, " which are always 

 grouped together" (n, p. 86). These two gemmules 

 differ in their spicules, the first having only curved 

 fusiformi-acerates ; the second having, 1, curved 

 fusiformi-acerates ; 2, attenuato-porrecto-ternates ; 3, 

 attenuato-bihamates or unihamates. And he considered 

 it " highly probable" that this difference in the struc- 

 ture of the gemmules is sexual. 



In 1872 Mr. Carter, having examined a bottle-full of 

 Tethya cranium which Dr. Bowerbank had presented 

 to the British Museum, believed that they were 

 divisible into two species, which he designated 

 " cranium" and " Zetlandica;" Dr. Bowerbank declined 

 (' Ann. Nat. Hist/ ser. 4, vol. x, p. 58) to acknowledge 

 the validity of the grounds on which Carter established 



