274 Rev. W. A. Leighton on the British Umbilicarise. 



superior cortex is formed of small polygonal cells intimately 

 united, and its superficial brown colour is veiled by a sort of 

 furfuraceous powder, whose cellular elements, very irregular, are 

 unequally distributed and variously associated. The cortical 

 layer of the inferior face of the thallus is about double the 

 thickness of the preceding, and forms with it nearly a fourth 

 part of the entire thickness of the lichen. This part of the 

 plant is greyish, of a horny consistence, and very hygrometric. 

 It is constituted, as nearly all the tissues of this sort, by glo- 

 bular utriculea, with extremely thick walls, and so united to 

 each other, that the external contours of each of them are in- 

 distinct. This horny layer bears on its free face an infinity of 

 minute papilljs of a conical or pyramidal form, and which are 

 continuous with it, that is to say, formed of a tissue entirely 

 similar, but of a very deep brown colour. The fibrous medulla 

 which occupies the middle of the thallus, is, as in most foli- 

 aceous lichens, a loose tissue filled with air, above which, the 

 spherical gonidia form a slight continuous layer. The structure 

 of the other species scarcely differs in any material point of view. 

 In our plate (PI. X. fig. 1) we have copied M. Tulasne's exquisite 

 illustrative section. 



The apothecia arise from the medullary layer, and their deve- 

 lopment appears to take place somewhat in the following manner. 

 In the spot where an apothecium is about to appear, the cortical 

 layer is, by the uplifting of the medullary layer, formed into a 

 small wart or tubercle. This tubercle opens in the middle, the 

 hymenium appears exposed to view, the cortical layer is on either 

 side thrown back or reflected upon itself so as to constitute a 

 kind of excipulum to the hymenium, which is gradually and 

 progressively protruded upwards by the medullary layer, until 

 a fully expanded apothecium is formed, sessile or closely ap- 

 pressed on the surface of the thallus. The apothecia are of a 

 deep black or brown colour, but a vertical section shows this 

 tint to be confined exclusively to the surface of the disk and of 

 the excipulum. The base of the hymenium is not subtended by 

 any carbonaceous mass, as in the Lecide^p, the medullary layer be- 

 coming in that part of a brown colour more or less deepened in 

 tint. The apothecia are either simple, forming a single patellula, 

 or compound, consisting of numerous gyrations having much the 

 general appearance of the lirella? in the Opegraphce. The internal 

 structure is the same in both cases ; and dissection shows that 

 the compound apothecia result, not from a division of the disk of 

 a single patellula, but from a great number of apothecia spring- 

 ing from the same spot, forced, by excessive compression against 

 each other and growth within a limited circular space, to assume 

 a gyrate direction, and to exhibit the singular appearances for 



