Notices of Memoirs— -A. G. Nathorst — Tracks of Animals. 409 



3. — Ansichten ueber die palaeozoischen Insecten, und deren 

 Deutung ; von Prof. Dr. Friedrich Brauer. Mit zwei photozinkogr. 

 Tafeln. 



This is an elaborate critical review of the classifications adopted 

 by Scudder, Brongniart, and others, for Palaeozoic insects, and of the 

 significance of the characters on which these have been based. In 

 many points the author disputes the views of Scudder, lately pub- 

 lished in Zittel's Handbuch der Pal aeon tologie. The wing-structures 

 of many of the Palaeozoic insects and of their nearest living allies 

 are well illustrated in the accompanying plates. Amongst other 

 conclusions, the author states that the Palaeozoic insects present no 

 contradiction to the views of biologists as to the origin of the class, 

 and that they did not form a special order which could be regarded 

 as a general basis for the existing orders of insects. 



4. — Bestimmung des specifischen Gewichtes von Mineralien, von 

 Dr. Victor Goldschmidt. 



The author points out the causes for the differences occurring in 

 the practical determination of the specific gravity of the same 

 mineral ; which rest not so much in the method adopted as in the 

 selection of the materials which are tested. G. J. H. 



II. — Notjvelles Observations sur des Traces d'Animattx et atjtres 

 Phenomenes d'origine purement mecanique decrits comme 

 "Algues Fossiles." Par A. G. Nathorst. Avec 5 planches 

 en phototypie et plusieurs figures intercalees dans le texte. Kongl. 

 Svenska Vetenskaps-Akademiens Handlingar, Bandet 21, No. 

 14. (Stockholm, 1886.) 



THIS memoir is intended as a reply to the objections raised by 

 the Marquis of Saporta and MM. Lebesconte and Delgado, to 

 the opinions previously published by the author, that many of the 

 supposed fossil algae are in reality nothing more than the tracks of 

 animals, or phenomena of purely mechanical origin. The fossils, 

 whose nature is thus contested, are commonly known as Cruziana. or 

 Bilobites, Harlania, Eopliyton. and some other genera. They generally 

 present themselves in clemi-relief on the under surface of the beds 

 in which they occur; no traces of organic substances are found 

 associated with them, and they are composed of the same minerals as 

 the matrix in which they are imbedded. The theory of their veget- 

 able character rests on the peculiarity of their markings, which are 

 supposed to be incapable of being produced by the tracks of 

 organisms. Dr. Nathorst, however, shows very conclusively, that 

 whilst it is difficult to understand how algae could thus form casts in 

 demi-relief on the under surface of the beds, such structures would 

 be the natural result of the filled-up tracks or burrows of marine 

 organisms. Of the manner in which these could be made, the author 

 gives practical proof by passing a movable roller, shaped like a 

 double spindle, over the surface of a layer of soft mud, and then by 

 means of gypsum obtaining moulds of the concave impressions. 

 Photographs of these moulds are given in the accompanying plates, 

 and they faithfully represent in almost every detail, the supposed 



