INTEODUCTION. 6 



Lithistid and Hexactinellid sponges, unlike those of the previous groups, are 

 more numerous and varied in the fossil than the recent state. The spicular com- 

 ponents of the skeleton in these sponges are firmly attached together, consequently 

 the form of the sponge is frequently preserved intact, even in cases where the spicules 

 themselves have subsequently been destroyed. The occurrence of detached spicules 

 and fragments of the skeleton scattered through the rocks plainly shows, however, 

 that only under favourable conditions of fossilization has the form of the sponges been 

 retained, and those now remaining probably comprise but a small proportion of the 

 number which previously existed. 



Fossil Calcareous sponges are abundant in certain strata, but they belong to a 

 family which difi'ers to such an extent from existing Calcispongia; that the relationship 

 has been greatly doubted. Recent discoveries, however, prove that the component 

 spicules in the fossil Calcispongise possess the closest resemblance to those of the 

 living examples of the order. 



Tn respect to their form and dimensions, fossil sponges present as great diversity as 

 living ones. The most prevalent form is like a cup, funnel, or vase ; but cylindrical, 

 club-shaped, fan-shaped, and branching examples are also very abundant. The 

 smallest complete fossil specimen which has come under my notice is a Calcisponge, 

 belonging to the genus Peronella, which measures 5 '5 millim. in length by 4 millim. 

 in widtli ; whilst the largest is a Lithistid, belonging to the genus Doryderma, which 

 reaches a length of 390 millim. by 1-35 millim. in diameter. 



Fossil sponges are of most common occurrence in calcareous and arenaceous rocks, 

 whilst they are rarely met with in shales and deposits formed from muddy sediments. 

 Strata containing masses and nodules of flint and chert frequently abound in fossil 

 sponges ; indeed there is every reason to suppose that the flint and chert itself is 

 derived from the dissolved skeletons of siliceous sponges. Both arenaceous and 

 calcareous deposits seem to have been favourable to the existence of Tetractinellid 

 and Lithistid sponges, for they occur alike in the Lower and Upper Green Sands as 

 well as in the Chalk. The Hexactinellid sponges, on the other hand, favour more 

 particularly the deeper-formed deposits of limestone and chalk, though rarely forms 

 of this order are present in the Upper Green Sand ; and, according to Manzoni*, they 

 are comparatively numerous in Miocene strata in Italy, associated with shallow-water 

 organisms. The Calcispongise, on the other hand, are most abundant in arenaceous 

 or shallow-water deposits, and thus in their habitats resemble the existing members 

 of the order. Exceptions, however, occur in this group, for several Calcisponges are 

 found in Jurassic limestones associated with Hexactinellids, and in one example in 

 the Museum a species of Peronella is growing attached to the surface of a Hexacti- 

 nellid sponge. 



At the present day, with the exception of the genus Spongilla, which inhabits fresh 



* Spugne silicee del Miocene medio, 



b2 



