MESOZOIC FAUNAS 177 



secrete siliceous skeletons in which agglutinated matter 

 is subordinate. The imperforate, porcellanous tests of 

 Miliolidae are often found, but vitro-calcareous Textu- 

 lariids, Lagenids and Globigerinids constituted the bulk 

 of the Mesozoic Foraminiferan fauna. Most types are 

 small, but such genera as Cristellaria and Frondicularia 

 often attained sufficient size in the Chalk to allow of 

 detection with the naked eye, even in the field. 



Radiolaria are far from abundant in British Mesozoic 

 deposits. From the records available in other districts, 

 it would seem that the Nasselaria, inferior to the 

 Spumellaria in older periods of the era, overtook and 

 almost outnumbered the more primitive types in the 

 Cretaceous. 



(C) PORIFERA 



Owing to the scarcity of deep-water deposits of 

 Mesozoic age, the Silicispongiae of that era are usually 

 less abundant than the Calcispongiae. Only in the 

 Chalk (in this country) can siliceous forms be called 

 common, but there they occur in such quantities as to 

 be responsible, in large measure, for the production of 

 flint. The semi-parasitic Monactinellid genus Cliona 

 has left its crypts in many Cretaceous shells (PI. iv. 

 fig. 5) ; Megamorine Lithistids, such as Doryderma, form 

 the nuclei of many curiously shaped flint-nodules ; 

 Tetracladine forms (e.g. Siphonia and Jerea) account 

 for much of the Upper Greensand chert; while the 

 exquisite tracery of Dictyonine Hexactinellids (ranging 

 back to the Trias) is spread over much of the Chalk in 

 beds of slow accretion. Coscinopora and Ventriculites 

 abound (as impressions) in the " rock-bands " of the 

 Upper Chalk, desilicified by percolating water. In many 

 cases their remains have been phosphatized ; in some they 

 have retained the more massive parts of their siliceous 

 12 



