ON FEMATOPIIYTON AND ALLIES. 39 



strong resemblance to true medullary rays. But a more critical examination discloses the 

 fact that, in their want of continuity, and in certain other respects to be referred to more 

 fully later, they possess peculiarities which are not consistent with true medullary rays. 

 The ti'ansverse section, also, shows certain lines which give rise to false layers, undoubt- 

 edly referable to alteration under conditions of pressure. In addition to these, it is not 

 difficiilt to also recognise certain fairly well defined layers, obviously due to dift'ereuces 

 of structure, and these layers, which are the " growth rings " of the original description, 

 do bear a certain resemblance to the growth rings of exogeus. Usually single (Plate I, 

 fig. 3), they frequently appear double (Plate I, fig. 2). In No. 5 they are by no means 

 concentric, but traverse the stem from side to side and find free terminations at the 

 periphery, where they coincide with the longitudinal markings already referred to — the 

 the more dense portions corresponding to the longitudinal ridges, while the less dense 

 parts coincide with the corresponding furrows. The peculiar disposition here noted is in 

 all probability referable to changes under pressure, whereby both an alteration of form 

 and a removal of the external layers were effected. 



In the larger and more highly silicified specimens, where the form of the trunk is 

 well preserved, the layers show a well-defined concentric arrangement, while the weath- 

 ered extremity of one specimen shows a very distinct protrusion of the denser portions of 

 the various layers, the less dense parts having weathered away precisely as occurs in 

 true exogens. 



Internal Stuxjcture. — A microscopical examination of transverse sections at once 

 renders it obvious that the appearance of layers referred to, is dependent upon variations 

 in density of structure. The inner face of each layer is composed of relatively large and 

 thick-walled cells, having a diameter ranging from 13.6 // to 34.6 /^ — the average size 

 approximating to the latter dimension. These cells are also separated comx^letely from 

 one another, often by intervals which considerably exceed the dimensions of the cells 

 themselves (Plate II, fig. 6). The outer face of each layer is composed of relatively small 

 cells, which range from 13.8 /< to 2t.6 /<, their average diameter api^roaching the former 

 figure. From this it is obvious that there is sufficient variation in structure to cause the 

 definition of layers, but when, in addition to this, we look for any abrvrpt transition from 

 the more dense to the less dense structure, sirch as commonly occurs in conifers and other 

 trees of exogenous growth, we find it wholly wanting. There is, in fact, no abrupt ter- 

 mination of the denser layer on its outer face, but the small cells merge somewhat gradu- 

 ally with the larger cells, in both an internal and external direction (Plate I, fig. 3 and 

 Plate I, fig. 2), so that were it not for the curvature of the layer itself, it would be very 

 difficult to determine to which of the less dense adjacent portions the more dense 

 tissue properly belongs. All transverse sections show, in more or less striking degree, 

 one feature of the structure which at once arrests attention. "While the plane of section 

 may cut the majority of cells at right angles, it is found to cut others obliquely, so as to 

 show a considerable portion of their length, thirs proving very clearly, as Mr. Carruthers 

 originally pointed out,' that all the cells do no not follow a parallel course, while it also 



' M. Mic. Jn'l viii, 164 ; plate xxxi, c. We may remark here that the figures given by Mr. Carruthers, in his 

 paper already cited, represent the various aspects of the structure in Prototasites with great fidelity, as will be seen 

 by comparison with the photomicrographs introduced into this paper. 



