16 Journal of the Mitchell Society \^Octoher 



(TMrsuta Dendy 1889, 1905 ; T.limicola Dendy 1905 ; T.cinachy- 

 roides Hentschel 1911). 



Tetilla and its relatives offer excellent illnstrations of the fact that 

 sponge genera become more and more difficult to distinguish as the 

 number of known species increases, and I venture to set down some 

 reflections on this matter. 



Each genus is, of course, only a group that has been gradually 

 built up round a type embodying a certain combination of well-marked 

 features or "characters." In the cases of the related genera Tetilla, 

 Tethya, and Cinachyra, the main "characters," grouped (Mendel- 

 wise) in pairs the members of which contrast, are as follows: 



1. A non-fibrous cortex (a), or a fibrous cortex (-4). 



2. Radiating cortical spicules not present (6), or present (B). 



3. Pores scattered (c), or grouped in poriferous depressions (C). 



Of these characters Tetilla typically embodies a, b, and cj Tethya, 

 A, B, and c; Cinachyra, A, B, and C. 



Comparison of related sponge species indicates that the characters 

 vary, more properly have varied during the evolution of the present 

 races, independently of one another, and hence the number of com- 

 binations actually found increases with the number of species that 

 come to be known in a genus. In the case of the above group of forms 

 there are more combinations than there are recognized genera, and 

 this is, of course, often the case in systematics. Thus among the 

 species congregated under Tetilla, we find not only the typical com- 

 bination but others as well: A, h, and c; a, h, and C. 



In such a state of affairs we may either combine genera until we 

 get groups so heterogeneous as to be useless to biology, or we can go 

 on splitting up genera on the plan that each genus shall represent 

 only a particular combination. Thus, on this idea, it would be 

 logical to divide the species, now grouped under Tetilla, into three 

 genera. 



But another difficulty faces us here in that the extremes of a char- 

 acter, the sharply contrasting conditions that constitute the members 

 of a pair, are so often connected by intergrades. Thus in Tetilla, 

 between non-fibrous cortex and fibrous cortex there are many inter- 

 grades. And it may confidently be said that the more intensive 



