INTEODUCTION. XIU 



does not establish genera, but it is, that man does 

 not always clearly comprehend them, and then wanting 

 industry to work out the problems, he substitutes his 

 own crude ideas in place of natm'e's arrangement. 



It appears that a genus should always have a struc- 

 tural foundation by which we are at once enabled 

 by positive and negative evidence to circumscribe our 

 field of examination, and to arrive all the more speedily 

 and certainly at our specific discriminations, and I am 

 the more confirmed in this opinion, as we have many 

 instances among the British sponges of natural genera 

 so striking in their structural characters as to be at once 

 recognised as such, by the most hasty observer, and 

 amongst the most obvious examples we may name 

 those of Di/sidea, Giocalypta, Geoclia and Tetliea. 



The course that I have pursued in working out the 

 species of the British sponges is just such as would be 

 adopted by a botanist desirous of working out the 

 specific history of a plant. The great division, whether 

 calcareous, siliceous, or keratose, must first be deter- 

 mined, and this is readily and easily to be known. The 

 genus is next to be ascertained, and as the genei^ic 

 characters are strictly confined to the organic struc- 

 tures of the skeleton and their mode of arrangement 

 in the body of the animal, a patient examination of a 

 section of the sponge made at right angles to its 

 surface, and mounted in Canada balsam, seldom fails 

 to lead to a correct determination of the genus ; but it 

 must be observed that the immersion of the slice of 

 the sponge under examination in Canada balsam is 

 essential to a successful result, as it is frequently the 

 case, that the large amount of sarcode, in specimens 

 immersed in water, renders the slender skeleton tissues 

 nearly or entirely invisible or so indistinct as to inevit- 

 ably lead to indecision or positive error, while the 

 mounting of the specimen in Canada balsam renders 

 the whole transparent, and enables us to discern with 

 certainty the structural peculiarities on which the 

 generic characters are founded. The genus being 



