242 
Slates. Unfortunately tliere is no dip to be made out in the 
Teesdale sections, and I have had no opportunity of examin- 
ing the exposures of Silurian rocks beneath the Permian 
escarpment, so as to be able to form an idea of the probable 
age of the Teesdale Beds. 
ON THE GENUS POTERIOCRINUS AND ALLIED FORMS. 
BY W. PERCY SLADEN, F.L.S., F.G.S. (PLATE X.) 
Having had occasion latterly to devote some attention to 
the different species of Crinoids which have been referred 
by various writers to the genus Poteriocrinus, the author 
is induced to lay the following notes before this Society, in 
the hope that they will prove interesting and useful to 
other members who may be occupied with the determination 
of our Yorkshire fossil Echinodermata. 
The basis upon which modern biological classification 
rests is so much more extensive than that which sufficed 
only a few decades ago, and the principles which are 
involved in the process are so much more fundamental and 
real, that no apology is requisite for undertaking all such 
necessary revision as the advance of knowledge imposes, 
pari passu with its own progress, upon the naturalist who 
conscientiously applies himself to the study of any special 
group of organisms. 
Furthermore, less hesitation is felt in bringing the results 
of this examination to notice, than might otherwise be the 
case, since the gTound which it is intended to cover has not 
been traversed for a considerable number of years, during 
which time, it is hardly necessary to say, very numerous 
additions have been made to the knowledge of the group 
both at home and abroad. 
