56 



B. Cefh. hypogena (occurring upon the lower surface of the 

 thallus) . 



i. Ceph. thalloidea (Peltidea venosa). 



2. Ceph. immersa (Nephroma and Lobaria). 



3. " placodioidea (Lecanora gelida). 



4. " granuliformia (Lecidea panaeola). 



5. " fruticulosa (Sticta amplissima). 



II. Pseudo-cephalodia (apparently not organically united with the 



lichen structure). 



As to the algal types occurring in the various cephalodia the 

 following groups are represented, according to Forssell : 



1. Nostocaceae. 



2. Stigonemaceae (Sirosiphon). 



3. Scytonemaceae. 



4. Chroococcaceae. 



5. Oscillariaceae. 



It is quite evident that this author has included under pseudo- 

 cepholodia many of the occurrences which I have included under 

 contingent symbiosis. It is also probable that some of his cephalodia 

 vera have no organic connection with the lichen structure and are 

 more likely to be forms of contingent symbiosis. Inasmuch as many 

 of the cephaloid structures are not sufficiently understood, I will now 

 give a more detailed description of undoubted cephalodia ; these may 

 readily be divided into two kinds, endotrophic and ectotrophic. 



(a) Ectotrophic Cephalodia. 



In general a cephalodium may be defined as an abnormal develop- 

 ment within or upon the lichen thallus induced by some foreign alga. 

 An ectotrophic cephalodium is one which occurs upon the thallus. 

 The form which has been more particularly studied occurs regularly 

 on Peltigera aphthosa (L.) Hoffm. In this species the cephalodia 

 occur as small granular or warty elevations upon the upper surface 

 of the thallus ; they are especially numerous toward the margin ; they 

 vary in size from very small, nearest the margin, to about the size 

 and form of a pin bead toward the center of the thallus. 



These cephalodia originate near the apex of the thallus. As far 

 as it has been possible to observe, they result from the development of 

 soredia (of some Eivularia-bea.r'mg lichen) which are carried to the 

 thallus, where they are retained by the short hyphal branches which 



