48 BRITISH FOSSIL SPONGES. 



Size. 



There is a great amount of variation in the dimensions of fossil Sponges, though 

 not reaching to the extremes present in living forms. The smallest fossil 

 examples at present known are calcisponges, of which two species have been 

 described, Blastinia pygmaa, H., and Peronella nana, H., varying from 2*7 mm. to 

 5*2 mm. in length, and 3*5 mm. in width; or about the size of peas. On the other 

 hand, cylindrical lithistid Sponges, belonging to Stichophyma tumidum, H., and 

 Doryderma Benetti, H., reach an extreme length of 390 mm. (15^ inches), and a 

 thickness of 125 mm. (5 inches). An open cup-shaped lithistid, Kalpinella rugosa, 

 H., is 110 mm. (4| inches) in height, and 230 mm. (9| inches) in extension at the 

 summit. Fossil hexactinellids do not attain the same large size as the largest 

 lithistids ; an example of Ventriculites cribrosus, Phill. sp., is 240 mm. in length by 

 55 mm. in width at the summit, and an imperfect specimen of Sporadoscinia capax 

 is 170 mm. in height by 168 in width at the summit. Fossil calcisponges are, as a 

 rule, smaller than those of other groups, but exceptional examples of Elasmostoma 

 and Pharetrospongia reach a length and width of 100 to 200 mm. (4 to 8 inches). 



Canal-structures. 



The canals in living Sponges form usually a complex system of anastomosing 

 tubes, of varying degrees of fineness, lined by epithelial membrane. In fossil 

 Sponges the canals consist of cylindrical channels bounded by the spicular tissues, 

 in the same manner as those shown in macerated and dried specimens of recent horny 

 and siliceous Sponges. In the fossil, as in the dried recent examples, all the finer 

 canals have disappeared, and only the course of the larger can be made out in the 

 skeleton. Further, in the fossil specimens the canals are not infrequently now 

 infilled with the calcareous or[ siliceous rocky material in which the Sponge is 

 embedded, and in some cases they are so filled with resistant silica that they appear 

 as a solid network of fibres standing out on the surface of the Sponge. In many 

 fossil Sponges, more particularly in those with a very open spicular skeleton, no 

 distinctive canal-system can be discovered ; and these forms are usually described 

 as being without canals, but there can be no doubt that such Sponges in the living 

 state possessed canals, the course of which, owing to the open character of the 

 skeleton, is not shown. In other fossil Sponges, the walls bounding the larger 

 canals consist of a distinct spicular membrane of a finer character than the 

 ordinary skeletal tissues of the body. 



In fossil Sponges, as in recent forms, there appears to be two distinct systems 



