GENERA AND SPECIES. 179 



VII I. COLLEMACEJS. 



This family is characterized by a foliose, dark-blue 

 thallus, which is highly gelatinous when moist. As a 

 rule, the thallus is thin and more or less lobate, at- 

 tached by an umbilicus. The surfaces are frequently 

 rugosely folded. Soralia are deficient. Rhizoids are 

 also deficient excepting in the genus Mallotium. The 

 algai are Nostoc, excepting perhaps in Hydrothyria. 

 In all cases they are bluish-green, and occur in chains. 

 This one character alone suffices to mark the repre- 

 sentatives of the family. Owing to the supposition 

 that the algae are not arranged in such well-defined 

 layers as in the other foliose forms these lichens are 

 spoken of as homoimerous. The algae are, however, 

 arranged in more or less definite layers, especially in 

 the groups above Collema. 



The apothecia are rather small, discoid and sessile 

 upon the thallus. Spores vary in form and septation, 

 and are mostly colorless. 



KEY TO THE GENERA. 



Cortical layers wanting 1. Collema. 



Cortical layers present. 



Rhizoids deficient, thallus thin. 



Lower surface of thallus not veined . 2. Leptogium. 

 Lower surface of thallus veined . . . 3. Hydrothyria. 

 Rhizoids long, numerous, thallus com- 

 paratively thick 4' Mallotium. 



1. Collema, 

 This genus is represented by many species. The 

 thallus varies from small and almost fruticose to quite 

 large and typically foliose. The amateur will have 



