BRITISH LICHENS. 



171 



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 Peltiform* 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 Solorina, presenting no margin ; or it may be a convex disc 

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

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

 in Nephronium. 



The lecanorine apothecium is so called because 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-like tubercle around it, at first entirely closed, afterwards open at 

 the summit, as in Thelotrema and Urceolaria (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 hypothecium 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 large genus Lecidea, hence the 

 name lecideine. It is sometimes also called patelliform. 



The UreUinef apothecium is that which forms an elevated ridge, 

 with an opening in the form of a slit running along the top. It prevails 

 in the pictorial Lichens, those which resemble writing, as Opegrapha and 

 Gr aphis. The receptacle here is usually black, and consists of a firm 

 cellular substance, and is generally called the perithecivm\. If the 

 perithecium entirely surrounds the thalamium, it is called entire (Fig. 20 ;) 

 if it be deficient beneath, it is called dimidiate. (Fig. 18.) The lirelhe 

 are very variable in form, being immersed or slightly elevated, broad or 

 nai'row, straight or curved, simple or branched. 



The pyrenodine§ apothecium is that in which the excipumm consists 

 of a closed receptacle composed of a dense leathery or brittle substance, 

 with a minute pore at the top caled the ustiolum\\. It may be globose or 

 conical, superficial, i.e., seated on the surface, or immersed in the thallus. 

 As in the lirelline apothecium, if it surround the thalamium it is called 

 entire, and if deficient beneath it is called dimidiate, and also has the 

 name perithecium applied to it. It is the characteristic form in 

 Vermcaria, approaching very closely the genus Spharia in fungi. 



* From pelta, Latin for target. 

 \ From lirella Latin for furrow. 



I From irepl, around, and Q^ktj, a Back. 



§ From pyren, Latin for kernel, or stone of fruit. 



II From ostiolum, Latin for a little door, 



