1 62 Rhcetic Fossils. 



Lamellibranchiate molluscs, Lima precursor very 

 much resembles Lima punctata of the Lias ; Monotis 

 decussata occurs at the top in thin limestone bands, 

 which some have considered to form part of the Lower 

 Lias. Ostrea fimbriata may possibly be 0. irregularis 

 of the Lias, but oysters are so variable in form that 

 they are of small value in such an inquiry. Pecten 

 Valoniensis, also a Khaetic shell, is a very variable 

 form. Plicatula intusstriata passes into the Lower 

 Lias. Anoplophora musculoides, another Rhaetic 

 shell, occurs with Monotis decussata in the thin bands 

 of limestone at the top, which some geologists call 

 Lias. Modiola minima is found both in the Rhaetic 

 and Lower Lias strata. Figures of some other well 

 known fossils are shown in fig. 34. 



All the Grasteropoda of the Rhaetic beds are said to 

 be peculiar to that formation, and the same is the case 

 with the fish ; for, many years a.go, Sir Philip Egerton 

 declared ' that the beds in question, hitherto considered 

 as belonging to the Lias, must be removed from that 

 formation, inasmuch as they present a series of fishes 

 not only specifically distinct from those of the Lias, 

 but possess, in the Granoid genera, the heterocerque 

 tail, an organism confined to the fishes which existed 

 anterior to the Lias.' l Of the Reptilia, Plesiosaurus 

 costatus, P. Hawkinsii, P. trigonus, and, according to 

 the late Mr. William Sanders, Ichthyosaurus platyodon, 

 are common to these Khaetic beds, and to the basement 

 beds of the Lower Lias. The discovery by Professor 

 Boyd Dawkins of the small Marsupial mammal Micro- 

 lestes antiquus, in the grey marls at Watchett, in 

 Somersetshire, is not without significance, for it speaks 



1 'A Notice on the Occurrence of Triassic Fishes in British 

 Strata; ' Proceedings of the Geological Society,' 1841, vol. iii., p. 409. 



