204 THALLOPHYTA. [CH. 



consequent on a particular manner of crystallisation of the 

 siliceous material, a matter of considerable difficulty. 



Suggestions as to the nature of Pachytheca have been 

 particularly numerous ; it has been referred to most classes of 

 plants and relegated by some writers to the animal kingdom. 

 The most recent addition to our knowledge of this problematic 

 fossil was the discovery of a specimen by Mr Storrie in which 

 the Pachytheca sphere rested in a small cup, like an acorn fruit 

 in its cupule. This specimen was figured and described by 

 Mr George Murray^ in 1895; he expresses the opinion that the 

 discovery makes the taxonomic position of the genus still more 

 obscure. Solms-Laubach briefly refers to Pachytheca in con- 

 nection with Nematophycus, and regards its precise nature 

 almost as much an unsolved riddle now as it was when first 

 discovered. For a fuller account of this fossil reference must 

 be made to the contributions of Hooker"'*, Barber^ and others. 

 The literature is quoted by Barber and more recently by 

 Solms-Laubach^. There are several specimens and microscopic 

 sections of Pachytheca in the geological and botanical depart- 

 ments of the British Museum. The genus has been recorded 

 from Shropshire, North Wales, Malvern, Herefordshire, Perth- 

 shire and other British localities, as well as from Canada; it 

 occurs in both Silurian and Old Red Sandstone rocks. 



Algites. 



A generic name for those fossils which in all probability 

 belong to the class Algae, but which, by reason of the absence 

 of reproductive organs, internal structure, or characters of 

 a trustworthy nature in the determination of affinity, cannot be 

 referred with any degree of certainty to a particular recent 

 genus or family. 



This term was suggested in 1894^ as a provisional and 

 comprehensive designation under which might be included such 

 impressions or casts as might reasonably be referred to Algae. 

 The practice of applying to alga-like fossils names suggestive 

 of a definite alliance with recent genera is as a rule unsound. 



1 Murray G. (953). 2 Hooker J. D. (89). -^ loc. cit. 



4 Solms-Laubach (95^) p. 81. ^ Seward (94^) p. 4. 



