27(> A MONOGRAPH OF THE 



inspection of their structural peculiarities enables us readily 

 to surmount the difficulty, for although the structure of 

 the skeletons very closely resemble each other, there are 

 very distinct differences in the length and proportions of 

 the spicula of which they are composed. The presence of 

 acuate spicula in I. varians readily separates it from the 

 other three, but in these the forms of the spicula are very 

 nearly alike. Their real differential characters exist in the 

 differences in their proportions. Thus, in I.permoUis we have 

 the spicula ^th inch in length, and 4 8 th inch in diameter ; 

 and in /. Peacliii^&t inch long, and ^rd inch in diameter ; 

 the latter agreeing nearly in length, but not having half 

 the amount of the diameter of the former. While in 

 I. cinerra the dimensions of the spicula are ^th inch in 

 length, and ^ 7 th inch in diameter, agreeing nearly in 

 diameter with those of /. permollis, but being almost half 

 as long again as the spicula of that species. 



The specimen sent to me by my kind friend Mr. 

 Peach was found attached to the stalk of a large Fucus at 

 Peterhead, North Britain. It is thirteen lines in length, 

 by seven in breadth, and about two in thickness. I have 

 also been favoured, bv the Rev. A. M. Norman, with the 

 loan of a specimen from Chudleigh Salterton, which agrees 

 in all its essential characters with that from Mr. Peach ; 

 but it is rather the thicker of the two, and has a brighter 

 and more decidedly yellow colour. Mr. Norman has also 

 obtained a specimen from Seahouse, County Durham. 



In one of Mr. Norman's specimens the gemmules are 

 abundant beneath the dermal membrane, they are more or 

 less spherical, and of a deep amber colour; one of them, 

 apparently a full-sized and matured one, was filled with 

 minute spherical vesicles. It measured ^st nicn in di- 

 meter. 



2. Isodu'TYa Peach ii, Bowcrbank. 



Sponge. Encrusting, surface smooth. Oscula simple, 

 minute, few in number. Pores inconspicuous. Der- 



