212 M/NERALS AND GEOLOGY 



Thallogens are composed of cellular tissue only, and in them there 

 is no proper (although sometimes a seeming) differentiation of stem 

 and leaf. They may be classed as Land Thallogens, growing in the 

 air ; and Aquatic Thallogens, growing in the water of seas, lakes and 

 streams. 



Land Thallogens comprise Fungi, which have no chlorophyll (green 

 colouring matter) in their tissues, and which require organized 

 matter for their nutrition ; and Lichens, most of which form incrus- 

 tations on stones and the bark of trees. Neither are of palseonto- 

 logical interest. 



Aquatic Thallogens may be regarded broadly as consisting of Algce 

 (ordinary sea-weeds and related fresh water confervse), and Micro- 

 phytes separating, under the latter designation, a group of very 

 minute or microscopic forms usually placed with the algae, and known 

 as diatomaceae, desmidiae, and volvocineae. 



Algce occur as fossil casts and impressions in rocks of all ages from 

 the Cambrian upwards ; but in the majority of instances fossil forms 

 can only be referred doubtfully to the special groups into which 

 modern algae are divided. They are commonly termed "fucoids." 

 and are abundant in our Silurian and other strata. Some of the 

 more common examples are shewn in the annexed figures : 111-115. 



FIG. in. FIG. 112. 



Lithrophycus Ottawaensis (Billings). Bythotrephis tennis (Hall). 



Trenton Formation. Medina and Clinton Formations. 



The remarkable impressions, commonly regarded as the tracks of 

 Crustacea, which occur in the Potsdam (Cambrian) formation of the 

 vicinity of Perth in Ontario, and in Beauharnois, Quebec, should 

 probably be referred to algae. These are known as Climactichnites 

 and Protichnites, figures 116 and 117. In Climactichnites, the form 



