146 THALLOPHYTA. [CH. 



evidence for including them among the select class of true 

 fossil algae. Sollas^ has shown that the structure known as 

 Oldhamia radiata is not merely superficial but that it extends 

 across the cleavage-planes. Oldhamia is recorded from Lower 

 Palaeozoic rocks in the Pyrenees'^ by Barrois, who agrees with 

 Salter, Goppert and others in classing the fossil among the 

 algae. The photograph accompanying Barrois' description does 

 not, however, add further evidence in favour of accepting Old- 

 hamia as a genus of fossil algae. 



The burrows made by Gryllotalpa vulgaris Latr., the Mole- 

 cricket, have been shown by Zeiller to bear a close resemblance 

 to a branch of a conifer in half-relief (fig. 30, 4), or to such a 

 supposed algal genus as Phymatoderma^. 



In fig. 30, 1, we have what might well be described as a fossil 

 alga. This is merely a cast of a miniature river-system such as 

 one frequently sees cut out by the small rills of water flowing 

 over a gently-sloping sandy beach. A cast figured and de- 

 scribed by Newberry as an alga, Dendrophycus triassicus^, from 

 the Trias of the Connecticut Valley, is practically identical with 

 the rill-marks shown in fig. 30, 1. The cracks produced in drying 



Fig. 31. Chondrites verisimilis Salt. Wenlock limestone, Dudley. From a 

 specimen in the British Museum (V. 2550). Slightly reduced. 



and contracting sediment may form moulds in which casts are 

 subsequently produced by the deposition of an overlying layer 

 of sand, and such casts have been erroneously referred to algal 



1 Sollas (86). 



2 Barrois (88). Keferences to other records of this genus may be found in 

 Barrois' paper. 



3 Zeiller (84). Phymatoderma is probably a horny sponge {vide p. 154). 



^ Newberry (88) p. 82, PI. xxi. There are some large specimens of this 

 supposed alga in the National Museum, Washington ; they are undoubtedly of 

 the nature of rill-marks. 



