540 DR. BOWKRBANK ON SILICEO-F1BROUS SPONGES. [June 20, 



the intermediate portions between the inhalant and exhalant surfaces 

 are so completely intermingled that it cannot be determined with cer- 

 tainty bow many of them there are. The fibres also of which they 

 are composed are very variable ; some of them, and especially the 

 largest ones, are nearly quite destitute of minute spinules, while others 

 of small diameter abound in them in an incipient state of development. 

 The central canals of the fibres are also very irregularly developed : 

 in some parts they are obsolete ; in others they are visible, but are 

 of extreme tenuity ; they rarely appear continuous for any distance ; 

 and frequently they terminate abruptly. The same irregularity 

 is apparent with the interstitial defensive organs. A few single 

 slender small attenuated spines are observable on the fibres of the 

 skeleton, disposed at considerable distances from each other ; they 

 are very variable in size and mode of disposition. A few rectangu- 

 lated sexradiate defensive organs are also present at considerable 

 intervals. They are mostly very minute, requiring a power of about 

 200 linear to render them distinctly to the eye; and if of larger di- 

 mensions, they are often abnormal and distorted in form. 



The prevailing irregularities in the structure of this species, in 

 addition to the more positive specific characters, will readily separate 

 it from other nearly allied species with which we are acquainted. 



EXPLANATION OF THE PLATES. 



Plate LVI. 



Fig. 1 represents a portion of one of the pieces of Beanea favoides, exhibiting 

 the dark sarcodous development of the skeleton on one part, and the 

 denuded rotulate skeleton on the other portion, x36 linear. 



2. A small portion of the confluent rotulate skeleton of the specimen repre- 



sented by fig. 1, showing the structure of the rotulse and the central 

 canals of the fibre, X80 linear. 



3. Farrea inermis : a portion of the skeleton, exhibiting its general struc- 



ture and the paucity of its armature, and a small piece of its mem- 

 branous structure, with numerous gemmules embedded upon it at a, 

 X 36 linear. 



4. The group of gemmules represented at a, fig. 3, X 80 linear. 



Plate LVII. 



Fig. 1 represents a portion of the skeleton of Farrea perarmaf a, with the manner 



of the disposition of its abundant armature, X01 linear. 

 2. A small portion of the specimen represented by fig. 1, exhibiting the 



structure of the defensive organs, X 123 linear. 

 3 represents a portion of the skeletou of Farrea irregularis, exhibiting the 



intricate and irregular structure of its rete, X31 linear. 

 4, a, b, c. Examples of the varieties in size and of the distortions of form 



of the interstitial sexradiate defensive organs, X 123 linear. 



