Sept.. 1S96 THE STRUCTURE OF THE GRAPTOLITES. 187 



consistence. After having been washed with water the rock may be 

 treated with hydrofluoric acid until the graptolite becomes free. 



From siliceous rock I have cleaned out graptolites with hydro- 

 fluoric acid, using the most concentrated, fuming acid, at a strength 

 of 55 per cent. Even from clay-slate itself graptolites may be cleaned 

 out with hydrofluoric acid, although for various reasons rarely with 

 good result. Cleaned out graptolites that are not to be further treated 

 ought, after having been carefully washed in water, to be kept in 

 glycerine in glass tubes stopped with corks and made air-tight with 

 gold size, asphalt varnish, copal varnish, or something of the kind. 



For examining the internal structure I have followed two 

 different methods. The one, followed chiefly when dealing with 

 Graptoloidea, consists of a kind of bleaching of specimens selected 

 for that pupose. This bleaching I at first brought about by means of 

 Schultze's maceration fluid, a re-agent long used by botanists, 

 consisting of strong or concentrated nitric acid and chlorate of potash 

 m solid shape. Subsequently, however, I have adopted another and 

 milder re-agent, eau de Javelle or potassium-hypochlorite. In spite of 

 the violence of the methods, one can, after some practice, venture 

 upon bleaching even when only one specimen is at disposal. After 

 bleaching, the graptolite should be washed in water and in alcohol of 

 gradually increasing strength. It is now devoid of colour, but not 

 clear. For clarifying, chloroform is best ; but often one can just as 

 well use turpentine, toluol, oil of cloves, or other ordinary clearing 

 fluid. Sometimes the graptolite is so colourless that it does not need 

 to be bleached, but may be clarified after simple passing through 

 alcohol. 



The other method consists of making series of sections of the 

 cleaned out graptolites. This method I have chiefly used for the 

 Dendroidea, which on the outside have a periderm so thick that the 

 thin inner walls would be consumed long before the outer periderm 

 had time to become transparent. The method for making series of 

 sections is the same as nowadays is used in most zoological 

 laboratories. The interpretation of a series of sections is considerably 

 facilitated by making a plastic reconstruction from it.^ 



The periderm may consist of pyrites or a yellowish brown, brown, 

 or black substance, that has been called chitin, and that probably 

 once consisted of some at all events chitin-like substance originating 

 in a way similar to true chiiin. I therefore consider the periderm as 

 the epidermis of the vanished animals. 



The Structure of the Graptolites. 



I. — Graptoloidea. 



In describing the structure of the Graptoloidea I shall begin with 

 those that are morphologically simplest, that is, with the Monograptidai, 



1 See Natural Science, vol. iii., p. 340, November, 1S93, and vol. vii., p. 379, 

 December, 1S93. 



