:E. a. N. Arber—The Triasstc Plant Yuccites. 11 



the second phase is propagated as a distortional wave, for, "among 

 material bodies, a solid alone really possesses the rigidity sufficient 

 for the production within it of transverse vibrations and for their 

 maintenance during their propagation." ^ But I have suggested an 

 alternative explanation of the two phases, depending upon the 

 presence of water-gas in the magma, which, from volcanic eruptions, 

 we know to be a constituent.- 



The caiise usually assigned for the supposed rigiditj'^ of the interior 

 is the enormous pressure to which it is exposed. It is held that 

 this would so increase the viscosity as to render it practically solid, 

 but I am not aware that any experimental fact has shown this to 

 be probable ; for it is a different (j^uestion from that of the melting 

 temperature being raised by pressure. We know that in tlie case 

 of a vapour there is a certain critical temperature, above Avhich no 

 pressure will reconvert it into a liquid. Is it not possible that in the 

 case of a liquid there may be a critical temperature above which no 

 pressure will convert the liquid into a solid ? And this may be the 

 case in the earth's interior. 



III. — Ox THE Affinities OF THE Triassic Plant Yuccites vogesiacus, 

 Schimper & Mougeot. 



ByE. A. Xeavell Arbek, M.A., F.L.S., F.G.S., Trinity College, Cambridge; 

 University Demonstrator iu Palaiobotany. 



IN a paper published in 1907, in the Transactions of the Linnean 

 Society of London,^ I described and figured, under the name 

 Zamites grcmdis, sp. nov., some excellent specimens of detached leaves 

 recently collected by my friend Mr. L. J. Wills, B.A., F.G.S., from 

 the Keuper rocks of Bromsgrove (Worcestershire). 



These fossils were regarded as identical with the specimens from 

 the Banter of the Yosges, first described by Schimper & Mougeot* 

 under the name of Yuccites vogesiacus. 



In discussing the affinities of this plant I pointed out ^ that this 

 question had been " a subject of much confusion in the past, and the 

 cause of considerable difficulty at the present ". The difficulty lay in 

 the fact that these large, lanceolate, Monocotyledonous-looking leaves, 

 with close, stout, and strictly parallel nerves, were almost invariably 

 detached, and with one possible exception, which, however, was not 

 very conclusive, there was no direct evidence, either from the British 

 or the Continental examples, as to the manner in which they were 

 borne. Consequently direct evidence as to their affinities was lacking. 



On the other hand, these leaves presented a very close resemblance 

 to the pinnae of a Zamitean frond except in size, in which respect they 

 were very dissimilar. I concluded, liowever, that they were the 

 detached pinna? of a Zamitean frond of large size. 



' Poincurc, The Xeiv Physics : International Scientific Series, 1907, vol. xc, p. 17;'. 

 ' Proc. Cambridge Phil. Soc. 



^ Arber, Trans. Linn. Soc. London, 1907, ser. ii, Bot., vol. vii, pt. vii, p. 109. 

 * Schimper & Mougeot, Monogr. Plant, foss. A'osges, 1844, p. A'l, pi. xxi, 

 figs. 1 and 2. 



•'' Arber, iliid., p. 115. 



