158 



STRATIGRAPHICAL GEOLOGY 



abundant in the Silurian period, about 100 species being known 

 in England belonging chiefly to the following genera, Cardiola, 

 Cardiomorpha, Ctenodonta, Orthonota, Gleidophorus, Grammysia, Am- 

 bonychia, Pterincea, Conocardium, Lunulicardium, and Modiolopsis. 



Gastropoda are fairly numerous, about sixty species being 

 known from the English Silurian. Most of them are referable to 

 the following genera : Pleurotomaria, Murchisonia, Bellerophon, 

 Platyschisma, Omphalotrochus (Horiostoma), Gydonema, Eunema, 

 Holopea, Holopella, Platyceras ( = Acroculia), and Subulites. The 

 last genus is noteworthy as possessing a channelled mouth, and 

 Murchisonia has a very short anterior canal, all the rest being holo- 

 stomous. 



Cephalopoda. These are also numerous ; but all the genera 

 belong to the Nautiloidea ; they include species of Orthocerat, 

 Phragmoceras, Gomphoceras, Trochoceras, Tretoceras, Ophidioceras, 

 Gyrtoceras, Actinoceras, and Ascoceras, the last being only known as 

 a Silurian form. 



Fishes. The earliest remains of vertebrate animals hitherto 

 found in Britain occur in the Ludlow Series, and are the remains 

 of fish belonging to the sub-class or order Ostracodermi. The chief 

 genera are Pteraspis, Gephalaspis, Auchenaspis, Scaphaspis, and 

 Eukeraspis. The sub-class Elasmobranchii is represented by Onchus 

 (spines) and shagreen-like plates referred to Coslolepis and Thelodus. 



The greater part of the Silurian System is capable of subdivision 

 into zones by means of the graptolite faunas, and though these are 

 not equally developed in all localities they always occur in the 

 same order of succession. No species have yet been recognised in 

 the Upper Ludlow Beds, but from the Aymestry limestone downward 

 the following zones have been recognised : 



Lower 

 Ludlow 



Wenlock 

 Beds 



Valentian 

 (Llandovery)* 



'Zone of Monograptus Leintwardiuensis. 



tumescens. 



scanicus. 



Nilssoni. 



vulgaris. 

 Cyrtograptus Lundgreni. 



rigidus. 



Linnarssoni. 



symmetricus. 



Monograptus Riccartonensis. 

 Cyrtograptus Murchisoni. 



, , Grayae. 



Monograptus crispus and M. exiguus. 

 Rastrites maximus. 

 Monograptus spinigerus and Cryptograptus conieta. 



, , gregarius. 



Diplograptus acuminatus and vesiculosus. 



