112 ■ Nicholson — On GraptoUtes. 



These curious fossils were considered by him to be the germs 

 or embryonic forms of Graptolites, and there can hardly be any 

 doubt as to the correctness of this conjecture. Numerous bodies, 

 very similar to those described by Hall, occur in the graptolitic 

 shales of Dumfriesshire,^ and have, in all probability, a similar 

 origin. It should be observed that the form and texture of these 

 germs constitute a strong argument against the conclusion that the 

 Graptolitidod are Polyzoa, and support those who wish to place the 

 Graptolites among the Sydrozoa, a view which is further sustained 

 by the occiirrence of forms like Corynoides, and by the discovery of 

 bodies resembling " ovarian vesicles," as noticed by Hall and by 

 myself, (see op. cit. supra, also the Geological Magazine, October, 

 1866.) 



It is, of course, in most cases impossible to refer the embryonic 

 Graptolite to any known adult form ; still, this can sometimes be 

 done with tolerable certainty. Thus, in the anthracitic shales of 

 Hart Fell, where Diplograpsus pristis is the predominant species, 

 there occur innumerable germs, which in their more advanced stages 

 appear referable to the above-mentioned species. Among these, 

 three main varieties present themselves, which, are probably, merely 

 as many different stages of development. The first and youngest 

 of these consists simply of a mucronate radicle, or initial point, 

 sometimes slightly dilated at its distal extremity (Fig. 21). When 

 further advanced the radicle is seen to be surmounted by an oval or 

 heart-shaped mass (Fig. 21 a), which, as a further step, becomes 

 gradually indented (Fig. 21 h), and is developed into the primary 

 cellules. When next seen the radicle is inconspicuous, and the 

 young form now difiers from the adult in size only. 



Anotlier very common form, which is certainly referable to some 

 di-prionidian species, consists of a pointed radicle, terminating in, or 

 giving origin to, a solid axis, the lower part of which is embraced 

 by two semi-circular lobes, which are so arranged as to alternate 

 distinctly with one another, leaving the upper part of the axis bare 

 (Fig. 22). When further developed, two additional lobes or cellules 

 are superimposed upon the first, the alternation being still main- 

 tained (Fig. 23). 



Other germs, again, appear to belong to mono-prionidian species. 

 Like the last, these seem to commence simply as a pointed radicle. 

 In the process of development a delicate solid axis is produced from 

 the apex of the radicle, proceeding from one side of it, and not from 

 the centre. From the base of this axis, and also on one side only, a 

 small semi-circular lobe, or cellule, is developed, being applied 

 partly to the axis and partly to the radicle (Fig. 24). No more 

 advanced stages of these unilateral, or mono-prionidian, germs have 

 as yet come under my notice. 



I The first notice of these embryonic forms, as occuring in Britain, came, I bolie-ve, 

 from Mr. W. Carruthers, Avho described and iigured a small germ, which he con- 

 sidered to be referable to his Diplograpsus tricorons. (See Annals and Magazine of 

 Natural History, Vol. iii. No. 13, p. 25). 



