HYMEDESMIA. G5 



publication in vol. ii of this work, a small angular stone 

 nearly covered by a thin cream-white incrustation, and 

 the remains of one of the valves of a bivalve shell, 

 the inner surface of which was nearly entirely coated 

 with the same sort of incrustation. He stated on labels 

 attached to them that they were from deep water at 

 "Wick, North Britain. The crust was spread over three 

 sides of the angular pebble so thinly that it was with 

 some difficulty that I could detach portions of it for 

 microscopical examination after having immersed it in 

 water ; its thickness not exceeding -^ ^y inch. AVhen 

 a portion of it is mounted in Canada balsam and 

 examined with a power of about 200 linear it becomes 

 a remarkably interesting object, abounding in strikingly 

 characteristic organs. The dermal membrane is so 

 thin and pellucid that it can only be recognised by the 

 small specks of extraneous matters adherent to its 

 surface, and neither oscula nor pores can be detected. 

 The minute hispidation of the surface can only be 

 perceived in portions of the sponge when mounted in 

 Canada balsam ; it is produced by the passage of small 

 portions of the distal extremities of the internal 

 defensive spicula through the dermal membrane. 



" The skeleton fasciculi are clearly unequally distri- 

 buted through its substance ; in some cases they are 

 grouped together rather numerously, while in others 

 they occur singly, or two or three of them are closely 

 adjoining each other ; and occasionally single spicula 

 are irregularly dispersed. The fasciculi are composed of 

 very slender acuate spicula, each fasciculus containing 

 a considerable number of them closely disposed in 

 parallel lines; but although thus slender they are 

 undoubtedly the legitimate representatives of the 



VOL. iv. 5 



