228 Mr. S. Bidwell. Effect of Magnetisation upon the [Mar. l r 



than one genus ; but, in absence of sufficient knowledge of the skull 

 to establish differences, the four species are referred to a new genus, 

 Diademodon. Its hinder molar teeth have undivided roots, and low 

 crowns, which are subquadrate or transversely ovate, surrounded by a 

 diadem of low cusps, which are chiefly developed on the external and 

 internal borders, with crenulations or minute cusps on the anterior 

 and posterior margins of these teeth. There is a low central cusp 

 in the middle of the crown from which slight ridges extend, chiefly 

 in the transverse direction ; but in the. type species these ridges 

 take the form of a cross. The species are distinguished by the form 

 of the crown and the details of the cusps. The upper and lower 

 teeth are opposed so as to be evenly worn, but the mandibular 

 teeth are narrower. 



These teeth are highly specialised, but distinct in plan from 

 Tritylodon, and from all known Reptiles. They closely approximate 

 to some of the higher Mammalia. The anthor refers Diademodon 

 to a division of the Theriodontia in which the teeth become worn 

 with use, which is named Gomphodontia. 



VIII. " On the Effect of Magnetisation upon the Dimensions 

 of Wires and Rings of Annealed Iron." By SHELFORD 

 BIDWELL, M.A., LL.B., F.R.S. Received February 14, 

 1894. 



In the year 1885 I submitted to the Royal Society the first of a 

 series of papers* npon the changes produced by magnetisation in the 

 dimensions of rods, Ac., of iron and other magnetic metals. The 

 chief, and perhaps the most interesting, subject of the paper was the 

 observation that if the magnetising force were sufficiently increased, 

 the extension which a magnetised iron rod at first underwent (a 

 originally noticed by Jonlef) was followed by contraction, the rod 

 ultimately becoming shorter than when it was nnmagnetised. The 

 elongation was generally found to attain a maximum with a magnet- 

 ising force of from 80 to 120 C.G.S. nnits, and to vanish with a force of 

 300 to 400, retraction occurring when still higher forces were applied. 



From that date until quite recently no accounts of similar experi- 

 ments by other workers have, so far as I know, been published. 

 About the beginning of last year, however, it was stated in the 

 scientific journals that M. Alphonse Berget had investigated the 

 magnetic dilatation of iron in strong fields, and had found that the 

 length of his bar was still increasing when the magnetic field had 



' Boy. Soc. Proc.,' vol. 40 (1886), pp. 109, 257 ; vol. 43 (1888), p. 407; vol. 47 

 (1890), p. 469 ; vol. 51 (1892), p. 495. ' Phil. Trans.,' vol. 179, A (1888), p. 205. 

 t Joule's ' Scientific Papers,' pp. 48 and 235. 



