2l6 



HISTORICAL PALAEONTOLOGY. 



Amongst the Mollusca we have to note the disappearance, 

 amongst the lower groups, of many characteristic Palaeozoic 

 types. Amongst the Polysoans, the characteristic "Lace- 

 corals," Fenestella, Retepora,* Synocladia, Polypora, &c., have 



Fig. 144. Triassic Lamellibranchs, a, Daonella (Halobia) Lommelli ; b, Pecten 

 Valoniensis; c, Myophoria lineata; d, Cardium RhcRticurn; e, Avicula contorta; 

 f, Avicula socialis. 



become apparently extinct. The same is true of many of the 

 ancient types of Brachiopods, and conspicuously so of the 

 great family of the Productida, which played such an important 

 part in the seas of the Carboniferous and Permian periods. 



B'nvlves (Lamellibranchiata} and Univalves (Gasteropoda} 

 are well represented in the marine beds of the Trias, and 

 some of the former are particularly characteristic either of the 



* The genus Retepora is really a recent one, represented by living 

 forms; and the so-called Retchorce of the Palaeozoic rocks should properly 

 receive another name (PJiyllopora), as being of a different nature. The 

 name Retepora has been here retained for these old forms simply in ac- 

 cordance with general usage. 



