56 PEOE. E. W. CLAYPOLE OI^T PTERASPIDIAN EISH IN 



more marked recognition than this from the systematist. It differ- 

 entiates the animals of this gronp from all the rest of the Vertebrata 

 possessing a hard skeleton. It may or may not be an indication of 

 approximation to lower and older forms. But in either case so 

 marked and significant a feature in the structure can scarcely 

 be satisfied with subfamiliar distinction in the classification, I 

 therefore propose to raise these aberrant forms, the number of which 

 is increasing, into a distinct family under the name of Pteraspidians 

 (Pteraspidse), and to limit the name Cej)halaspid8e of Professor Huxley 

 to the forms composing Professor Lankester's group of Osteostracans. 



2. Tlie StratigrapJiical Horizon of PalsDaspis. 



In the correlation of the Pennsylvanian strata here concerned 

 with those of England, there is fortunately little difficulty in estab- 

 lishing certain definite horizons in both, which from a palseontological 

 point of view may be regarded as equivalents. As may be seen in 

 the table (p. 59), the English Lower Ludlow lies on the horizon of the 

 "Water Lime of Pennsylvania and New York. This correlation rests 

 mainly on the presence and first appearance in both of huge Crus- 

 taceans of the types of Euryjpterus and Pterygotus. On both sides of 

 the Atlantic this horizon is especially characterized by these forms. 

 On both sides they suddenly attain in this stratum very great pro- 

 minence, and from it they rise into higher, but are searely known in 

 lower beds. 



The Lower Ludlow contains the oldest vertebrate fossil yet known, 

 the single specimen of Scaphasjpis ludensis found in 1859. The 

 Water Lime of Middle Pennsylvania has yielded no remains of fish, 

 and it lies about 300 feet below the place of the locally absent 

 Corniferous Limestone, the lowest American fish-bearing horizon. 

 Below the Water Lime lies the great mass of coloured shale forming 

 the medial part of the fifth group of Eogers (Geol. Survey of Penns.). 

 This is divided in Middle Pennsylvania as follows : — 



feet. 



The Grey Shale 200 



The Variegated Shale 700 



The Eed Shale 700 



At the top of the second division lie some thin beds of variegated 

 Sandstone which I have named " The Bloomfield Sandstone." In 

 this bed were found the fossils here described. It is exposed near 

 iN'ew Bloomfield, Perry Co., Pennsjdvania. 



In a paper recently read before the American Philosox^hical Society 

 and printed in their Proceedings for 1884, I have discussed the evi- 

 dence for the age of the " Eifth group " of Pennsylvania. Space 

 will not allow the reproduction of the argument here, but in that 

 tract I have shown that the shales above mentioned are the equivalent 

 or, rather, the continuation of the Onondaga Shale of New York, a 

 series immediately underlying the Water Lime. 



On this view of the correlation of the English and American 

 strata, the Onondaga Salt group has no equivalent in England. It 

 is so represented by Sir R. Murchison (Siluria, p. 472), who in this 



