LINDSAY, ON ABROTHALLUS. 41 



The deformations of the thallus of Parmelia conspersa, on 

 which A. Ocvysjjorus grows, as examined in specimens from 

 Barmonth, communicated by Mr. Leighton, more resemble 

 those of C. glauca than those of P. saxatiUs. In Welsh speci- 

 mens of ^. Smithii and A. oxysporus, growing on furfuraceous 

 forms of P. saxatilis, the portions of thallus on which the 

 parasites occur have more the character of the ordinary laciniae 

 of P. saxatilis than in Scotch specimens. The thallus of 

 P. saxatilis in these specimens is whitish, fi'iable, and mealy, 

 compared with Scotch ones, which are tough and coriaceous. 

 This difference is probably to be accounted for by differences 

 in the geological character of the habitat. 



In addition to the other irregularities of structure or ap- 

 pearance already alluded to, I have occasionally observed, 

 particularly in Higliland specimens, the cortical layer of the 

 deformed thalli partially or wholly eroded, apparently by 

 insects, the subjacent white or medullary tissue being thereby 

 exposed. This erosion sometimes appeared like a cross section 

 of a number of plicse on the same squamule, or of a series of 

 superimposed squamules or lobes. It occurs sometimes in 

 similar localities in the ordinary thaUus of P. saxatilis, as 

 well as on its anamorphoses. Frequently the medullary 

 tissue exposed is of a brilliant saffron-yellow. I have fre- 

 quently noticed on damp walls a similar condition of Parmelia 

 parietina, gi\'ing the thallus and apotliecia a white-variegated 

 appearance. 



The origin and mode of growth of the globular deformations 

 of the thallus of P. saxatilis and Cetraria glauca above 

 described, appear to me to throw a new light on the nature 

 of the var. concentrica of P. saxatilis (Leiglit. 'Lich. Brit, 

 exsicc.,' No. 232, fasc. 8, 1856), lately found by Sir W. C. 

 Trevelyan, rolbng freely before the wind on the exposed 

 sheep-walks or chalk downs of Dorsetshire, and particularly 

 on Melbury Hill, near Shaftesbury.* The characteristics of 

 this curious lichen are, its erratic nature, its globiilar form, 

 and its want of adhesion to any base of support. Its ex- 

 ternal appearance, as well as its characters on section, are 

 very similar to those of the lead-coloured or grayish, globose 

 anamorphoses of the thallus of P. saxatilis before referred 

 to. Sjjecimens of the latter, if detached and rolled about on 

 the surface of the ground in a similar way, would undoubtedly 

 acqmre a similarly globose form. This erratic form of P. 



* Proceedings of the Botaii. Society of Edinburgli, in ' Scottish Gar- 

 dener,' March, 1856, p. 100; and Notes by Rev." M. J. Berkeley, in 

 ' Gardener's Chronicle,' Feb. 9th, 1856, p. 84, and March 15th, 1856, p" 172. 



