124 LYCOPODIALES [CH. 



results of unequal decay in the more superficial tissues, have 

 necessarily led to the preservation of the same specific type 

 under a variety of forms. 



Our knowledge of the anatomy of Lepidodendron stems 

 enables us to recognise in fossils of very different appearance 

 specimens in various conditions of preservation of one and 

 the same type. Such names as Knorria, Bergeria and Aspi- 

 diaria are examples of generic titles instituted before any 

 adequate knowledge of Lepidodendron anatomy was available. 



Differences in age as well as different degrees of decortica- 

 tion have contributed in no small measure to the institution of 

 generic and specific names which more recently acquired know- 

 ledge has shown to be superfluous. 



a. Knorria. 



The designation Knorria, after a certain G. W. Knorr of 

 Ntirnberg, was proposed by Sternberg in 1826^ for casts of 

 Palaeozoic stems of a type figured more than a century earlier 

 by Volkmann^. Goeppert, in his earlier works, published draw- 

 ings of fossil stems which he referred to Sternberg's genus : one 

 species he at first called Didymophyllum Schollini. He after- 

 wards' described some specimens which showed that the 

 features characteristic of Knorria may occur on partially de- 

 corticated stems with leaf-cushions of the true Lepidodendron 

 type. His specimens, preserved in the Breslau Museum, 

 demonstrate the accuracy of his drawings and conclusions. 

 Goeppert, and after him Balfour*, drew attention to the different 

 appearances presented by branches of Araucaria imbricata when 

 preserved with the surface intact and after partial decortication, 

 as illustrating possible sources of error in the determination of 

 fossil stems. 



Although it is now a well-established fact that fossils bear- 

 ing the name Knorria are imperfect lepidodendroid stems, the 

 use of the term may be conveniently retained for descriptive 

 purposes. The specimen from the Commentry coal-field of 



1 Sternberg (26) A. 2 Goeppert (52) A. p. 196. See also Kidston (01) p. 50. 



■' ibid. (52) A. p. 44. Pis. xxx. xxxi. Lief. 1 and ii. '' Balfour (72) A. 



