Dr. B. S. Traquair — On a Neio Fossil Shark. 3 



Magazine for 1877, p. 441, as being more numerous than those 

 of the ordinary type. A little consideration will show that the two 

 types of faulting might be produced in one and the same bed. 

 Suppose a given stratum of some thickness to be bent into a curve. 

 A state of tension will arise on the outside, and one of compres- 

 sion on the inside. The two spaces in which these opposite con- 

 ditions exist will be separated by a neutral plane.' If the strains 

 be relieved in both cases by faulting — they may of course be 

 relieved in other M^ays — then ordinary faults w^ill be produced on 

 the outside and reversed faults on the inside. 



It would be extremely interesting to study these faulted masses 

 in the field, and to trace the connection between the deformations 

 which they have suffered and the moi"e powerful earth-movements 

 that have affected the rocks. Until this has been done, there are 

 many points that must of necessity remain more or less obscure. 



Of course the importance which the reader will attach to the 

 present instance will depend on the view he holds as to the extent 

 to which the general principles of rock deformation are illustrated 

 by minute examples. For my own part, I must confess to a growing 

 conviction that the essential points of both mountain- and low-land 

 stratigraphy, to borrow expressions used by Prof. Lapworth, may 

 be frequently studied in hand specimens. 



n. — Description of a Fossil Shark [Gtenacanthus costellatus) 

 FROM THE Lower Carboniferous Rocks of Eskdale, Dum- 

 friesshire. 



By E. H. Traquair, M.D., F.E.S. 

 (PLATE II.) 



r|"1HE deficiency of our knowledge of the organization and con- 

 JL figuration of the Palaeozoic Selachii is an unfortunate fact too 

 well known to biologists to render it necessary for me to dilate upon 

 here. Immense numbers of genera and species have been founded 

 upon detached teeth and spines ; but as yet very few specimens have 

 occurred, which threw any light upon the general organization and 

 configuration of the ancient possessors of these now scattered relics. 



Among the many new and remarkable fossil fishes, which the 

 Lower Carboniferous beds of Eskdale, Dumfriesshire, have recently 

 yielded to the investigations of the officers of the Geological Survey 

 of Scotland, as well as of other collectors, none are more interesting 

 than an entire specimen of a fossil shark, acquired a short time ago 

 by the British Museum. It was found at Glencartholm by Mr. Jex, 

 collector to Mr. Damon, of Weymouth, and for the privilege of 

 describing and figuring it, I am indebted to my friend Dr. Woodward, 

 F.R.S., Keeper of the Geological Department. 



The specimen is represented in PI. II. Fig. 1 (reduced three- 

 fourths), and presents us with a somewhat gracefully shaped fish, 

 laterally compressed, with two dorsal spines, and a heterocercal tail. 



1 See A. Helm, Mechanismus der Gebirgsbilding, Band II. s. 17. 



