Review of Recent Geological Literature. 59 
animal organism those parts which are important for his purpose, and 
concentrates his attention on them. These are the bones, the teeth, and 
the skin, and appendages if hardened as is frequently the case. I) is 
from these, that the palaeontologist must study the vertebral life of tin- 
past for seldom have the soft parts left any impress or record of their 
existence. 
In dealing with the fishes, the classification of Prof. Huxley is pre- 
ferred to others that have been proposed as more in accord with the sys- 
tems adopted in other classes of Vertebrata. This places the Cyclosto- 
mata by themselves. These however, being unknown in a fossil state 
require no notice. 
Dealing with their geological age the author remarks that being the 
lowest of the Vertebrata, their remains would naturally be looked tor in 
rocks of very early date. This expectation is fulfilled. "The earliest 
known fishes in Britain occur in the Lower Ludlow group of the Silur- 
ian. " The Paloeaspis of Pennsylvania however, from the Onondaga 
group represents an older date, probably coeval with the Wenlock, while 
the Onchus pennsylvanicus of the same state, from the Clinton, corre- 
sponds in age with the upper part of the Llandeilo or the "May-hill" 
sandstone. These indicate a rather greater antiquity for both ganoids 
and elasmobranchs. 
The elasmobranchs, have been a very important order from early 
times. But as their skeletons are and were almost entirely cartilagin- 
ous we are compelled to depend on the evidence of teeth, spines and der- 
mal scutes for their restoration with the aid of an occasional and unique 
specimen where an impression of some parts of the body may have been 
preserved. 
A singular fact in this connection is the frequent occurrence of thin 
beds composed almost entirely of the teeth, bones and scutes of these 
and other fish. Such are the well-known Ludlow bone-bed of Silurian 
age, and the Rhnetic bone-bed in England, both of which though only 
- an inch or two in thickness are remarkably persistent. The Delaware 
bone-bed of Corniferous age in Ohio is another case in point. They are 
apparently beach-lines or wash-deposits and indicate a much greater 
abundance of fish-life at those eras than the ichthyic remains found 
elsewhere would lead us to suppose. At the same time we must not for- 
get that in naming genera and species from detached teeth and spines, 
the paleontologist is often obliged to represent several parts of the same 
animal by distinct names, and the mistake is often made by those who 
are unfamiliar with his material of supposing that each of these 
names is intended to connote a separate organism. So far is this from 
the truth, that the term species has in palaeontology in many cases an 
entirely arbitrary meaning and implies no belief in any connection such 
as is intended in recent biology. For the progress of science it is neces- 
sary to name these fossils, but tin; palaeontologist holds all such names 
at their real value and waits till some lucky find shall enable him to 
put together two or more as only the separate parts of a single being. 
