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ness and structure. As a rule, it is thicker, as is well shown in 

 Omphalaria , Leptogium and other genera. This is for the purpose 

 of affording the additional mechanical support required, as well as 

 of aiding in the ejection of spores. The cells of this layer retain 

 the same relative position they had in the thallus ; so that the ends 

 which pointed upward in the thallus now point downward or diagon- 

 ally outward. 



The four layers described always occur in apothecia with typi- 

 cally developed thalloid exciples. The algal layers never occur in 

 the so-called " excipulum proprium," or proper exciple. The medul- 

 lary and the cortical layers may be present, but even these reach 

 their highest development in the apothecia with thalloid exciples. 

 We can readily understand the structure and development of the 

 thalloid apothecium by comparing it with the thallus of the flattened 

 fruticose lichens — Cetraria, for example; in this group we find that 

 the thallus contains the following layers: i, cortical; 2, algal; 3, 

 medullary; 4, algal; 5, cortical. That is, the apothecium presents 

 a "centric" thalloid structure, if we compare epithecium, thecium 

 and hypothecium to one of the cortical layers of the thallus ; its 

 mode of development is likewise essentially the same as that of the 

 centric fruticose thallus. The absence or presence of aigal layers 

 in the apothecia leads us to recognize two types which we shall dis- 

 cuss somewhat in detail. 



2. TYPES OF APOTHECIA. 



(«) The Fungal Type. 

 This type occurs in nearly all crustaceous lichens and in some of 

 the foliose forms. The apothecium contains no algae and in the 

 lowest form no part of the thallus enters into its formation ; in all 

 essentials it is like the apothecia found among the Ascomycetes. It 

 consists of a thecium and hypothecium ; the margin of the hypo- 

 thecium forms the proper exciple ; hypothecium and exciple form 

 the perithecium in the closed apothecia,. The term perithecium is 

 not employed in reference to open discoid apothecia. 



(5) The Thalline Type. 

 This always contains symbiotic algae ; its structure has been suf- 

 ficiently explained in the discussion of the apothecia. 



