274 Rev. W. A. Lcighton on the British Unibilicarire. 



superior cortex is formed of small polygonal cells intimafely 

 united, and its snjierlieial ))ro\vn colour is veiled by a sort of 

 furfuraceous jiowder, whose cellular elements, very irregular, are 

 unequally distributed and variously associated. Tin; cortical 

 layer of the inferior face of the tliallus is about doubh- tiie 

 thickness of the j)reecding, 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, l)y glo- 

 bular utricules, 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 papilhe 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 f(jrm a slight continuous layer. The structure 

 of the other .sjK-eie.s 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 aj)othecia arise from the medullary layer, and their deve- 

 lopment ajjpears to take place somewhat in the following manner. 

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

 layer is, by the u[)lifling 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 cxci])ulum to the hymenium, which is gradually and 

 progressively protruded upwards by the niedullaiy layer, until 

 a fully expanded apotheeium 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 Lcciderp, 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 ])atellula, 

 or compound, consisting of numerous gyrations having much the 

 general aj)j)earance of the lirelhc in the Opeyraphce. The internal 

 structrure 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 



