Prof. E. W. Claypole — FUh- Remains from the U. Silurian. 519 



Britain is situated, is at present the most glaciated hemisphere. As 

 it is very difficult to conceive that the earth had at any former period 

 a lower initial temperature, or that the sun possessed less heating 

 power, glaciation in the North could never have depended upon the 

 conditions argued in Dr. Croll's theory. The author suggested that 

 glaciation within latitudes between 40° to 60° was probably at all 

 periods a local phenomenon depending upon the direction taken by 

 aerial and oceanic currents ; as, for instance, Greenland is at present 

 glaciated, Norway has a mild climate in the same latitude, the one 

 being situated in the predominating Northern Atlantic currents, the 

 other in the Southern. Certain physical changes suggested in the 

 distribution of land would reverse these conditions, and render 

 Greenland the warmer climate, Norway the colder. 



20. — On some Eemains of Fish from the Upper Silurian Eocks 



OF Pennsylvania. 



By Professor E. W. Claypole, B.A., B.Sc. (Lond.), F.G.S.; 



of the Second Geological Survey of Pennsylvania. 



THE earliest vertebi-ate animals yet known from any part of the 

 world are some remains of fish in the Upper Silurian rocks of 

 England.^ They are for the most part of three types. First, short 

 fish-spines, named by Agassiz Onchus tenuistriatus ; second, fragments 

 of shagreen, or the skin of a placoid fish {Thelodus and Sphagodus), 

 belonging probably to the same that carried the spine ; and third, 

 ovate, finely-striated plates or shields, supposed to be the defensive 

 armour of some fish, unlike any now living. 



No one has doubted the ichthyic nature of the first and second of 

 these three forms. But as regards the third there has been much 

 controversy. Evidently allied to CejjJialaspis, its right to the name 

 of fish has been called in question, and suspicion has been raised in 

 regard to the whole family of the Cephalaspids. 



On the whole, however, it seems best to retain them in the class 

 of fishes, and to this conclusion Professor Huxley evidently inclines 

 in the conclusion of his " Essay on the Classification of the Devonian 

 Fish." One may expect some, or even considerable, divergence of 

 structure from the usual ichthyic types in such early forms. 



These English fossils occur in the lowest beds of the Devonian 

 (Cornwall), and in the highest beds of the Silurian (Shropshire and 

 Hereford). The well-known Upper Ludlow " bone bed " has yielded 

 them in considerable quantity, and one specimen is reported by Sir 

 C. Lyell in his "Elements of Geology " (1865) as discovered from 

 the Lower Ludlow, beneath the Aymestry limestone. Below this 

 horizon 1 have never heaixl of their occurrence. 



The English Ludlow, taken as a whole, has been usually correlated 

 with the Lower Helderberg of North America, and on good grounds, 



1 The oldest known Fossil Fish-remain in Britain was obtained by Mr. J. E. Lee, 

 r.G.S., in 1859, from the Lower Ludlow Beds, of Church Hill, Leintwardiue, 

 Shropshire (see Lankester's Mon. Foss. Fish. Pal. Soc. 1867, p. 2Sf). It is referred 

 to the genus Hcaphaspis (see Annals and Mag. Nat. Hist. 1859, p. 45, and " Geologist," 

 1860, vol. iii. p. 79). 



