106 THE RH^TIC PERIOD. CHAP. 



Dimyaria. Area, mentioned by Dr. Wright. 



Cardium rhseticum. Merian. 



Cardium cloacinmn. Quenst. 



Pullastra arenicola. Strickland. 



Pleurophorus elongatus. Moore. 



Unio. Brodie. 

 FISHES. The names from Agassiz. 

 Acrodus minimus. Garden Cliff. 

 Ceratodus altus. Garden Cliff. 

 Nemacanthus filifer. Garden Cliff. 



monilifer. Garden Cliff and Combe Hill. 



Hybodus minor. Garden Cliff and Combe Hill. 

 Gyrolepis Albert! Garden Cliff and Combe Hill. 



tenuistriatus. Garden Cliff and Combe Hill. 

 Saurichthys apicalis. Garden Cliff and Combe Hill. 



REPTILES. 



Ichthyosaurus. 



Plesiosaurus. Garden Cliff and Wainlode. 



A larger series of fishes and reptiles occurs at Aust Passage, on 

 the Severn, near Bristol, where, in particular, the teeth of Ceratodus 

 were plentifully found by Mr. Higgins. The reptilian remains are 

 very much scattered and often in fragments, much as these and 

 other fossils often occur about the junction-line of deposits, where 

 a remarkable change of life occurs ; for example, the bone-bed in 

 the Upper Silurian and another at the base of the mountain lime- 

 stone. 



On considering this list with attention we shall perceive no 

 Cephalopod, no Gasteropod, no Brachiopod, no Echinoderm, no 

 Coral. The shells, though few as to species, are found profusely 

 in certain narrow zones ; they are marine, probably belong to shallow 

 water, subject to occasional disturbance during periods of lengthened 

 tranquillity. The disturbances are marked by sandy deposits and 

 detached remains of vertebrata during longer periods : at intervals 

 shoals of small conchiferous mollusks existed. In the upper layers 

 of Monotis decussata and Myacites musculoides the whole period of 

 life of a mollusk is represented ; embryonic, young, and full-grown 

 specimens occurring on a plane surface ; while the shells of Pullastra 

 arenicola are of nearly uniform age. Pyrites is gathered abundantly 

 about the bones, and is scattered in some of the shales. 



The general aspect of the fauna is both liassic and Keuperian ; 

 the reptilia are more liassic, the fishes are more Keuperian. Some 

 of the older life remains, some of the newer life has come into view. 



