CHONDROSIA. 73 



and a thickness of 10-14 millim. ; smaller specimens are proportionately 

 thinner. 



The colour is very constant ; it is always in reflected light dark bluish black, 

 and in transmitted light dai'k brown. The surface is shiny, but not quite 

 smooth. A I'eticulate structure, as described by 0. Schmidt in Chondrosia 

 plebeja, does not exactly make its appearance, but still the roughness is of a 

 kind not met with in other Clwndrosia species, so that by this alone our species 

 can be distinguished. A transverse section shows that the cortex is highly 

 developed ; it has a dark outer margin and is light coloured towards the interior. 

 The pulpa is of a uniform dark brown colour ; a few canals are seen, but 

 these are rare and small. The oscula are numerous and grouped together ; 

 they possess small " chimneys," thin membranous frills which are about 

 1 millim. high, simple, cylindrical, with a smooth margin. 



The canal-system shows some peculiarities which distinguish it from other 

 species. It may perhaps eventually be advantageous to establish a new genus 

 for this species for this reason. 



Scattered all over the surface we find small pores, measuring about O'Ol 

 millim. ; these are circular, and situated in groups of 5-10, and there is always 

 a group of such pores at the base of each concavity of the rough surface. 

 Below each group of pores we find an extensive subdermal cavity, which is con- 

 nected with adjacent cavities of a similar kind by tangentially extended, lacunose 

 canals. Two layers of canals can be distinguished. The canals of the outer 

 layer communicate with each other by means of very rare and minute pores, 

 and with the larger lacunes below by means of similar pores, which in conse- 

 quence of their paucity and smallness are difficult to find. All these lacunes 

 lie so close together that only narrow walls of tissue remain between them. 

 The average diameter of the lacunes in the outer layer is 0*05, of those in the 

 lower layer 0*2 millim. ; the whole lacunose layer has a thickness of 0*4 millim. 

 Below this no lacunae are found in the cortex, except a few very distant and 

 large irregular tangential canals, which collect the water from the small com- 

 municating outer lacunae ; these canals have very irregular transverse sections 

 and an average diameter of 1 millim. The cortex is also 1 millim. thick, so that 

 the whole thickness of it is taken up by the canals ; the lower side of these 

 large tangential canals, from which numerous inhalant canals originate, lies 

 in the surface which divides the cortex from the pulpa of the sponge. The 

 canals which descend from the large superficial tangentials are cylindrical 

 and follow a more or less radial direction ; repeatedly ramifying, they 

 become smaller the further they penetrate into the sponge. The ciliated 

 chambers surround their final caecal narrow ramifications. They are spherical, 

 have a diameter of 0*05 millim., and do not appear to be very numerous. 

 The inhalant pores could not be found ; the exhalant openings are 

 small and circular. Eegarding their shape and position they do not differ 



