52 A MONOGRAPH OF THE 



Johnstonia, but having subsequently consulted with my 

 friend Dr. Johnston, of Berwick-on-Tweed, on the pro- 

 priety of making it the type of a new genus, I afterwards 

 named it Pachymatisma. 



The outer surface of the crustular dermis is quite smooth ; 

 those parts of it in littoral specimens which are most 

 exposed to the light, are of a dark gray colour, sometimes 

 approaching black, while the marginal portions near the 

 base are frequently of so light a gray as to be nearly white. 

 The interior substance is of a dull yellow colour, firm in 

 texture, and very much resembling the crum of bread in 

 appearance. 



The thickness of the dermal crust varies from a quarter 

 to half a line. It is composed of ovaria, closely embedded 

 in membranous structure; the greater portion of them 

 appear to have ejected their prolific contents and become 

 solid, but near the surface single specimens or small groups 

 are filled with this substance. Intermixed with the ovaria, 

 there are also frequently to be observed round siliceous 

 molecules, of about the same diameter or rather less, than 

 that of the adjoining spicula. 



The dermal membrane is thin, and aboimds with short, 

 stout, fusiformi-cylindrical spicula, which are more or less 

 irregularly tuberculated j they are also dispersed abun- 

 dantly on the membranes throughout the whole of the 

 crustular dermis, and are found occasionally in the sarcode 

 of the interstitial membranes, lining the excurrent canals. 

 Their average dimensions are, length jj^th in., diameter " 

 sjLgth inch. 



There is great variety in the form of the spicula, espe- 

 cially in those connecting the crustular dermis with the 

 body of the sponge. The normal form of the connecting 

 spicula appears to be attenuato-expando-ternate, the radii 

 being more or less acutely terminated, and having a long 

 attenuating shaft terminating acutely. In other cases they 

 are cylindro-expando-ternate, and frequently very stout in 

 their proportions; and between these two forms every 

 imaginable variety and malformation may be found, the 

 radii being frequently bifurcated or contorted to a great 



