lxxvi BRITISH GRAPTOLITES. 



of the views then held by himself and others as to the nature, structure, repro- 

 duction, classification, etc., of the Graptolites. The Introduction is divided into 

 nine chapters. 



Chapter 1. — Historical Notices, summarising the progress of research among the 

 Graptolites from the time of Linnaeus to 1872. 



Chapter 2. — Form and Mode of Preservation of Graptolites. — In discussing the 

 mode of nomenclature to be employed in describing a graptolite, Nicholson here 

 first suggests the title of " polypary " for the whole of the graptolite skeleton. 

 He adheres to the old term " calycles," or " cellules," declining to adopt the term 

 " hydrothecse." 



The various states of preservation in which Graptolites occur are described. 

 The carbonaceous material of the rock in which they are generally preserved is 

 considered to be of "animal origin," and can " hardly be ascribed to anything else 

 except the Graptolites themselves." 



Chapter 3. — General Morphology of Graptolites. — In this chapter Nicholson 

 describes in considerable detail the structure of a typical Monoprionidian and 

 a Diprionidian form, and the various aspects — " profile," " axial," and " scalariform " 

 — under which they may be seen. 



Chapter 4. — Special Morphology of Graptolites. — (1) Solid Axis. The hollow 

 character of this structure previously suggested by him is again asserted, and 

 this time with more certainty. The curious tube-like rod in the Rhabdopleura 

 he considers " lends great support to this view." This also explains the apparent 

 ability of the axis to grow independently of the rest of the polypary, and to 

 prolong itself distally as in species of Diplograpsus. The distal extension of 

 the axis is very rare in Mouograpsus, and never exists in Didymograpsus, Goeno- 

 grapsus, etc. He considers that the axis is never present in the shape of a "thin, 

 flat, apparently double plate" as believed by Barrande (' Grapt. de Boheme,' p. 4), 

 and Hall ('Grapt. Quebec Group,' p. 22). 



The character and position of the axis in Phyllograpsus, Betiolites, and Trigono- 

 grapsus are discussed ; the fact that the axis is inside and not outside the polypary 

 in the two latter genera is insisted upon, while the absence of any axis whatever 

 in the Dendroidea is remarked. 



(2) Surface Markings and Ornamentation of the Polypary. — The striae observed 

 running parallel with the aperture of the cells are for the first time described as 

 "growth lines," and good figures are given of the "pustules" visible at the base 

 of the cell walls, especially in M. vomerinus. 



Chapter 5. — Nature and Function of the Base in the Graptolites. — (1) Radicle. 

 The form of the so-called "radicle" or "initial point" as then accepted is more 

 clearly represented in this work than in previous papers, and it is distinguished 

 from the "radicle" as referred to by Hall, by which he meant merely the com- 

 mencement of the solid axis. Nicholson, however, did not realise the invariable 



