1 88 INVERTEBRATE PALAEONTOLOGY 



zone in that stage. Among Pteriidae, Pseudomonotis is 

 almost a rock-former in the Rhaetic, while Pteria, 

 abundant in that period, is common in the Lias and 

 Lower Oolites. Oysters belonging to all genera re- 

 cognized in that protean group occurred, and abounded, 

 throughout the era. Ostrea itself is a rock-former in 

 the Lower and Middle Lias, Great Oolite, Kimmeridge 

 Clay and Purbeck beds. Gryphaea is one of the best 

 known fossils of the Lower Lias, and is common in the 

 Oxfordian. Alectryonia is especially characteristic of 

 the Oolites, but occurs abundantly in the Cretaceous ; 

 while Exogyra is almost restricted to the Upper Oolites 

 and Lower Cretaceous. In the Lower Greensand, 

 specimens of Exogyra sinuata more than a foot in 

 length are not uncommon. Cardinia, a very common 

 fossil of the Lower Jurassic, was possibly a descendant 

 of the Carbonicola-^Q^\i of Carboniferous date. In 

 estuarine facies of the Lower Oolites, and again in the 

 deltaic marls of the Wealden, Unio, hardly to be dis- 

 tinguished from the fresh- water Mussels of existing 

 rivers, occurred in some abundance. Lastly, Trigonia 

 (see PL iii. figs, i and 2), sprung from Schizodus of the 

 Permian (through the Triassic Myophoria), can be 

 collected in hundreds at many Jurassic horizons, and 

 maintained its abundance in the Lower Cretaceous. 

 On the whole, there is a tendency for Lower Jurassic 

 Trigonias to have linear ornament (e.g. T. striata, PI. xiv. 

 fig. i), while those from the Upper Oolites have pustular 

 surfaces (e.g. T. clavellata, PI. xiv. fig. 2). Morpho- 

 genetic phases showing derivation of the latter from the 

 former type can be studied on well-preserved shells from 

 high horizons. Three families of Isodonta were particu- 

 larly prominent in the Mesozoic. Pectinidae abound 

 in the Middle Lias and in most clear-water deposits 

 of later date. In the Upper Cretaceous the subgenera 



