182 ANATOMY AND PHYSIOLOGY 



"TETHIE (Tetliea). 



" Polypier tubereux, subglobuleux, tres fibreux mterieure- 

 ment ; a fibre subfasciculees, divergentes ou rayon- 

 nantes de 1'interieur a la circonference et agglutinees 

 entre elles par un pen de pulpe ; a cellule dans un en- 

 croutement cortical quelquefois caduc. Les oscules 

 rarement perceptibles." 



Dr. Johnston's version of the generic characters differs 

 slightly from Lamarck's. They are as follows : 



" Sponge tuberous, suborbicular, solid and compact, invested 

 with a distinct rind or skin, the interior sarcoid loaded 

 with crystalline spicula collected into bundles and 

 radiating from a more compact nucleus to the circum- 

 ference. Marine." 



It is nruch easier to find faults in the generic characters 

 of both the authors quoted, than it is to improve them. 

 The extreme simplicity of the structural characters of Tetliea 

 is a strong temptation to endeavour to multiply them ; but 

 in doing so, Dr. Johnston has introduced two the struc- 

 ture of the dermal portion of the sponges, and the tuberous 

 nature of its surface which are not common to all the 

 known species. If we consider the word " tuberous " in 

 the usual English acceptation of the word, as a body " full 

 of knobs or swellings," then very few or perhaps none of 

 the species of Tetliea would, in their natural condition, 

 exhibit this character, but all of them would be in a greater 

 or less degree subglobular. Dr, Johnstone's description of 

 Tetliea was founded on the structure of T. lyncwrium only, 

 and in this species the " thick rind ' is very distinctly to 

 be seen, but in other species this structure is totally 

 wanting. It therefore ceases to be of value as a generic 

 character, and becomes a specific one only. Under these" 

 circumstances I propose the following modification of the 

 previously published generic characters : 



