THE TRIASSIC PERIOD. 217 



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 

 found in the Triassic strata of almost all regions. These 

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

 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, c\ belonging to the Trigoniada, 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 

 characterized by the occurrence in them of the Scallop, Pecten 

 Valoniensis (fig. 144, fr) ; the small Cockle, Cardium Rhaticum 

 (fig. 144, d} ; and the curiously-twisted Pearl-oyster, Avicula 

 contorta (fig. 144, e) this last Bivalve being so abundant that 

 the strata in question are often spoken of as the "Avicula 

 contorta beds. " 



Passing over the groups of the Heteropods and Pteropods, 

 we have to notice the Cephalopoda, which are represented in 

 the Trias not only by the chambered shells of Tetrabranchiates, 

 but also, for the first time, by the internal skeletons of Dibran- 

 chiate forms. The Trias, therefore, masks the first recognized 

 appearance of true Cuttle-fishes. All the known examples of 

 these belong to the great Mesozoic group of the Belemnitida; 

 and as this family is much more largely developed in the suc- 

 ceeding Jurassic period, the consideration of its characters 

 will be deferred till that formation is treated of. Amongst the 

 chambered Cephalopods we find quite a number of the Palae- 

 ozoic Orthoceratites, some of them of considerable size, along 

 with the ancient Cyrtoceras and Goniatites; and these old types, 

 singularly enough, occur in the higher portion of the Trias 

 (St. Cassian beds), but have, for some unexplained reason, not 

 yet been recognized in the lower and equally fossiliferous 



