26 BRITISH LICHENS. 



coloured. We have finally to point out the excipulum, or receptacle 

 within which the previously described parts of the apothecium are 

 contained. In the species under examination this is formed by a 

 continuation of the substance of the thallus, which has taken the shape 

 of a miniature cup, in the bottom of which the hymenium and the 

 hypothecium are seated. (Figs. 11, 12.) If a perpendicular section be 

 made through the middle of the apothecium, canying it down tlu-ough 

 the thallus, there will be no difficulty in seeing, with a pocket lens, the 

 an-angement of all the parts in the order we have here described them. 

 It is only in certain genera that the excipulum is formed of the same 

 tissue as the thallus. It is often formed of its own special tissue, in 

 which case it is called a proper excipulum ; and the margin it forms 

 when looked at from above is called a proper margin, whereas in the 

 former case it is called a thalline excipulum and a thalline margin. 



There is yet one more term used in reference to the apothecium, 

 which it is necessary to explain — the epitheeium.* This term is used to 

 denote the upper surface of the hymenium, the part that is presented to 

 the hght. 



The various parts of the apothecium we have described are essen- 

 tially the same in all, though assuming a multitude of different forms ; 

 hence the student must learn to recognise them however they present 

 themselves in nature. To aid him in this we will point out some of the 

 more striking modifications of the apothecium. 



The Peltiformf apothecium, is so called because of its resemblance to 

 a small shield. It may be in the form of a depression, sunk into the 

 thallus, as in So'orina, presenting no margin ; or it may be a convex disc 

 arising from the upper surface of the thallus, and having a margin, as in 

 Peltigera. It occurs in some genera on the under side of the thallus, as 

 in Nephronium. 



The lecanorine apothecium is so called becaiise it is the charac- 

 teristic form in the genus Lecanora, and, as we have seen in the specimen 

 under examination, has an excipulum formed from the substance of the 

 thallus. It may be innate or immersed in the thallus, which forms a 

 wart-hke tubercle around it, at first entirely closed, afterwards open at 

 the summit, as in Thdotrema and Urcenlaria (Fig. 19 ;) or it may be raised 

 above the general surface of the thallus on a short stem-like base, as in 

 Physcia, Parmelia, and Lecanora. (Figs. 11, 12.) 



The lecideine apothecium is that in which the tissue of the excipu- 

 lum is essentially different from that of the thallus, being, like the 

 preceding forms, either sunk into the thallus or seated on its surface. 

 (Figs. 21, 17.) In this form of apothecium the excipulum is a continua- 

 tion of the hj-pothecium as already described, or perhaps more properly 

 the latter is a continuation of the former, presenting no very well defined 

 line of separation. It prevails in the lai-ge genus Lecidea, hence the 

 name lecideine. It is sometimes also cailed patelliform. 



* Prom eirt, upon, and OriKTj, a sack, 

 t From peita, Latin for target, 



