SPONGIDA. 



149 



with tubercles and intervening depressions, by means of 

 which they are interlocked into a rigid framework. The 

 agreement in shape between the skeleton -spicules of Si- 

 phonia, and those of the living Lithistid genus Discodermia 

 (fig. 45, A), is so close, as shown by Mr Sollas, that it will 



Fig. 45. A, A single skeletoii-spicule of the living Lithistid Discodermia polydiscits, magni- 

 fied 60 diameters ; B, Small portion of the skeleton of Siphonia pyriformis, similarly enlarged. 

 (After Sollas.) 



be perhaps with difficulty that the latter can be retained 

 as generically distinct from the Cretaceous type. Other 

 well-known Cretaceous Lithistids belong to the genera Che- 

 nendopora, Jerea, Chonella, Verruculina, Astrocladia, Amphi- 

 thelion, &c. In the Tertiary period, lastly, numerous Lithistid 

 Sponges have been detected, but none of these are sufficiently 

 important to demand special consideration. 



