94 Mr. T crner's Description s of 
small. It is now many years since I first found L. hit co-alb us on 
elms at Acle in Norfolk; Mr. Dickson showed it to me growing 
plentifully on lime trees at Croydon, and I have lately found it 
on willows at Coltishall. 
4. Lichen porriginosus. 
L. crust.a, tenui pulverulenta albo-virescente : scutellis fuscis ; ju- 
nioribus niveo-marginatis concavis, adultioribus tuberculifor- 
mibus. 
Tab. VIII. Fig. 4. 
Innascitur ulmi montanae cortici apud Caistor prope Yarmouth, 
Crustam habet tenucm, pulverulentam, sparsam, e granulis minu- 
tissimis, globosis, neutiquam cohaerentibus, constantem ; sic- 
cam albam, madid am pallide virescentem, Byssumque botiy- 
oidem valde simulantem. Scutellae huic insident rarae, subro- 
tundce, magnitudine fere Ervi seminis, initio concavae, disco 
fuscae, margine niveae ; progrediente aetate fiunt planae, mox 
convexae et tuberculiformes, marginis omnino expertes. Made- 
factae ceraceam quandam et fere subdiaphanam praestant spe- 
ciem ; siccatae atro-fuscae evadunt. 
The situation which naturally belongs to this Lichen, at least 
among the British species, is between subfuscus and vernalis, with 
both which it has points of striking affinity. Its shields in colour 
approach nearly to those of the former, but differ in regularly 
assuming, as they grow old, the shape of tubercles ; and, still 
more strongly, in their border, while young, being of a snowy 
white, and of a substance quite dissimilar to the crust; whereas 
the scutellae of L. subfuscus are for the most part more concave in 
age than in youth, and their margin always appears not only to 
be homogeneous with the crust, but also in general to be a mere 
elevation 
