18 USNEEI. 



tinous development an inordinate expression, in our next ; the 

 last group of foliaceous Parmeliacei COLLEMEI. Like the Pan- 

 nariei, with which it is most intimately associable, this family 

 descends to very humble, and even crustaceous forms ; and thus 

 .anticipates, though in another line of direct affinity, the nor- 

 mally crustaceous LECANOREI. These, while ascending, now 

 conspicuously, into conditions recalling the higher, foliaceous 

 Parmeliacei, run yet into others wherein at last the thallus 

 becomes wholly subordinate, and the fructification, as in the 

 lowest lichens, plays the principal part ; exhibiting an extraor- 

 dinary variety of modification, and anticipating, not seldom, 

 types only fully exemplified in other tribes. And yet there is 

 no doubt that Parmelia and Lecanora may be looked at as mem- 

 bers of a continuous series ; and some of the most extreme of 

 Lecanoreiue deviations from the tribal type (as Pertusaria and 

 Thelotrema) revert yet, in certain instances, to conditions which 

 we cannot well compare with anything remote from Lecanora. 



Fam. 1. USNEEI. 



Thallus erectish, typically fruticulose, and passing then, 

 not seldom, into much elongated, pendulous forms; vari- 

 ously also now dilated, and at length also depressed, or sub- 

 foliaceous. 



Though well distinguished, as a whole, from the next suc- 

 ceeding family, which is typically horizontal and foliaceous, the 

 latter also ascends, in all its most important divisions, into fru- 

 ticulose states, to be discriminated carefully from the typically 

 vertical Usneei. It is easy however to discern what is really the 

 preponderant affinity of most of these ascendant Parmelieine 

 lichens : as of Theloschistes clirysoplithalmus to T. parietinus ; 

 of Parmelia Camtschadalis to P. Icevigata ; or of Physcia ciliaris, 

 and, especially P. leucomela, etc., to P. speciosa. But, on the 

 other hand, the family now before us is represented at its centre 

 by a genus ( Cetraria) in which a certam degree of dilatation of 

 the frond is all but everywhere discernible ; and, in this genus, 

 we find finally (in some of our most familiar rail-lichens) so near 

 an approach, in habit, and even in character, to Parmelia, that 

 one may well hesitate to which group a lichen shall be referred : 

 and the difficulty will only yield to a fuller knowledge of the 

 whole differentiation of the two series of forms. 



