dergo a change in color, usually becoming grey, brown or dark 

 brown, especially over the apothecia ; as in Trypethelmm, the thallus 

 never becomes visible to the naked eye. The hypothecium and ex- 

 ciple are quite constantly dark in color, consisting of a network of 

 hyphae with dark cell-walls. The paraphyses are colorless, shorter 

 than in Trvpcthclium, slender, with a decided tendency to become 

 gelatinous or granular. The spore-sacs are quite slender and cylin- 

 drical. 



The spores are usually more or less colored and constantly four- 

 celled, otherwise resembling those of the preceding genus ; some 

 forms seem to have spores devoid of color ; it may be, however, 

 that these represent immature conditions. It should also be remark- 

 ed that the young, undeveloped spores of TrypcthcUum are four to 

 six-celled and also that all the younger spores of Pyrenula are co- 

 lorless. 



The representatives of this genus are southern in their distribu- 

 tion, though quite a number occur in our territory; they live upon 

 the smoother barks where they form colonies of considerable size. 

 In their habit they do not seem to differ materially from the closely 

 related fungi (Sphaeriaceae). The symbiotic relationship is as yet 

 in a rudimentary stage ; it would be interesting to determine if any 

 of the Pyremilas can develop apothecia without the symbiotic algae. 



PLATE 52. 

 PYRENULA NITIDA Ach. 



1. Plants natural size, a, apothecia. 



2. Single apothecium enlarged. 



3. Section of apothecium. 



4. Algae and hypae 



5. Paraphyses and spore-sac. 



6. Spores. 



3. COXOTREMA Tuck. Syn. Lich. 186. 1848. 



The present position of the genus is very doubtful ; it has no 



characters which will give it a definite position in any of the nine 



families. Its apothecial characters place it nearest the Verrucaria- 



ceae ; the algae are, however, foreign to this family, for instead of 



Chroohpus they are Cystococcns. Exceptions of this kind occur 



elsewhere and therefore need not be considered of special impor- 



