46 PORIFERA. CALCAREA 



Rhaphidonema. Cup- or funnel-shaped or leaf-like, usually 

 with definite canals. Oscula on either the inner or the outer 

 surface. Spicules of three rays, one of which is but slightly 

 developed. On one (or sometimes both) surfaces is a thin, compact 

 or finely porous layer of spicules. Trias to Cretaceous. Ex. 

 R. macropora, Lower Greensand. 



Barroisia. Usually compound and bushy. Individuals cy- 

 lindrical, each divided into a series of chambers by transverse 

 partitions, which have a central circular opening, through which 

 a tube usually passes. Canals simple, numerous, minute. Spicules 

 slender, three-rayed ; also a surface layer of larger spicules. Lower 

 Greensand to Chalk. Ex. B. anastomans, Lower Greensand. 



PorosphaBra. Small simple sponges, commonly more or less 

 spherical, but sometimes pear-, thimble-, or melon-shaped ; often 

 free, but sometimes attached to foreign bodies. Numerous, simple, 

 straight, radiating canals open at the surface by minute apertures. 

 Spicules with four rays, of which three are short and blunt and fused 

 to the rays of adjoining spicules, whilst the fourth ray is longer and 

 tapering. A surface layer (not often preserved) consists of a mixture 

 of minute three- and four-rayed spicules and simple rods. Upper 

 Cretaceous. Ex. P. globularis, Chalk. 



Distribution of the Porifera. 



The Sponges are all aquatic, and with the exception of 

 the Monaxonid genus Spongilla and its allies, all marine. 

 They are found in the seas of all parts of the world and are 

 more numerous between the shore-line and 200 fathoms 

 than at greater depths; many of the genera have a 

 very wide distribution. All the Orders except the Octac- 

 tinellida and the Heteractinellida have living repre- 

 sentatives. The Monaxonids are abundant between the 

 shore-line and 200 fathoms, and gradually decrease in 



