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 Productidce, which played such an important 

 part in the seas of the Carboniferous and Permian periods. 



Bivalves {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 

 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 



, f. Triassic Lamellibranchs. a, DaoneUa (Halobia) Lommelli ; i>, Pecten 

 Valoniensts; c, Myophoria lineata; d, Cardiutn Rlueticum ; e, Avicula contorta; 

 f, Avicula socialis. 



found in the Triassic strata of almost all regions. These 

 groups belong to the family of the Pearl-oysters (Aviculida), 

 and are singular from the striking resemblance borne by some 

 of their included forms to the Strophomeiuz 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, c\ 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 "Rhaetic" or " Kossen" beds are 



