96 Bone Beds. 



most of the latter being new. There are 7 species of 

 Pteropoda of 3 genera. It is remarkable that the 

 described Gasteropoda only amount to 15 genera and 

 52 species, while in Bohemia, in equivalent rocks, 200 

 species, some years since, are mentioned by Barrande, 

 and as many of the Brachiopoda and Lamellibranchiate 

 molluscs. Of the Nucleobranchiata 10 Bellerophons 

 are known, and 7 genera and 53 species of Cephalopoda, 

 among which the genus Orthoceras decreases from 42 

 to 35 species. 



Near the top of the Upper Ludlow strata there 

 are several thin bone beds, containing teeth and 

 scales of small Placoid fish of the genera Onchus (?) 

 Sphagodus, and Thelodus. At present these are the 

 oldest known fishes. They are found in strata which 

 contain several species of the remarkable crustaceans 

 Pterygotus and Eurypterus, some of which are small in 

 size, while the largest Pterygotus, discovered by Dr. 

 Slimmon near Lesmahagow, in the uppermost Silurian 

 rocks, attained about 9 feet in length. The very 

 uppermost Silurian beds in England sometimes, as, for 

 example, near May Hill, contain the remains of land- 

 plants, consisting of small pieces of undetermined twigs, 

 and the sporecases of one of the Lycopodiacese Pachy- 

 theca spherica. 



In these last-named facts there is much significance, 

 bearing on the physical geography of the next so-called 

 geological epoch, which will be explained in the sub- 

 sequent chapter. In the meanwhile I may remark, that 

 I use the words so-called geological epoch, in the same 

 sense that the words epoch or period are employed in 

 dealing with civil or political events. Taking the 

 geological world, and the civil world, which deals with 

 the history of man as parallel, there is no break in time 



