THE TRIASSIC PERIOD. 



211 



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

 ancient types of Brachiopods^ and conspicuously so of the 

 great family of the ProductidcB, which played such an important 

 part in the seas of the Carboniferous and Permian periods. 



Bivalves i^Laniellibranchiatd) and Univalves {^Gasteropoda) 

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

 some of the former are particularly characteristic either of the 

 formation as a whole or of minor subdivisions of it. A few of 

 these characteristic species are figured in the accompanying 

 illustration (fig. 144). Bivalve shells of the genera Daonella 

 (fig. 144, a) and Halobia {Monotis) are very abundant, and are 



Fig. 144. Triassic Lamellibranchs. a, Daonella {Halobia) Lotizmelli; b, Pecten 

 Valoniensis ; c, Myophoria lineata; d, Carduim Rhcetictmi; e, Avicula contorta; 

 /, Avicjila socialis. 



found in the Triassic strata of almost all regions. These 

 groups belong to the family of the Pearl-oysters {AviadidcE), 

 and are singular from the striking resemblance borne by some 

 of their included forms to the Strophomence amongst the Lamp- 

 shells, though, of course, no real relation exists between the 

 two. The little Pearl-oyster, Avicula socialis (fig. 144, /), is 

 found throughout the greater part of the Triassic series, and is 

 especially abundant in the Muschelkalk. The genus Myo- 

 phoria (fig. 144, f), belonging to the Trigoniadce, and related 

 therefore to the Permian Schizodus, is characteristically Trias- 

 sic, many species of the genus being known in deposits of this 

 age. Lastly, the so-called " Rhsetic " or " Kossen " beds are 



