STUDY OF THE GRAPTOLITES. 201 



enlarged, form elongated sacs with swollen extremities, which are finally 

 dehiscent ; and then, as I suppose, discharging the ovules or germs, are 

 gradually absorbed or dissipated. 



Although these sacs are distinctly defined, they liave scarcely any 

 apparent substance, except along the lateral margins, which are limited by 

 a filiform extension resembling the solid axis of a graptolite. There are 

 likewise numerous fibres of this kind traversing the sacs ; and these some- 

 times remain attached to the original stipe after the other parts are 

 separated. In one example, we have conclusive evidence that they are 

 connected with the solid axis of the parent stipe. The gradations of deve- 

 lopment in these sacs may be studied in figures 6-9, Plate i. 



In the specimen fig. 10 of the same plate, the ordinary cellules are 

 removed, and the fibres are still seen joined to the axis, showing the origin 

 of the reproductive sacs. In most specimens bearing these sacs, the 

 cellules of the stipe are so obscure that the species cannot be determined ; 

 but in fig. 9 we find them attached to a well-marked stipe of G. whitjiddi. 



This mode of reproduction in the graptolites shows much analogy with 

 the Hydroidea, and would indicate the sertularians as their nearest 

 analogues.* 



Upon the surfaces of the slate where these bodies occur, there are 

 numerous graptolitic germs, or young graptolites of extremely minute 

 proportions, ranging from those where the first indications of their form 

 can be discovered, through successive stages of development till they have 

 assumed the determinate characters of the species. 



In several examples, these minute germs have been detected near to and 

 in contact with the reproductive sac ; and in one case, there is but a hair's- 

 breadth between one of the fibres of the sac and one of the oblique 

 processes at the base of the germ. It cannot be said that we have 

 detected the germ actually within the sac ; but the numerous young indi- 

 viduals lying near them, and upon the surfaces of the same laminae, off"ei- 

 very good arguments for supposing that they have been thus derived. 



The earliest defined form which we observe in the young graptolites 

 consists of the initial point or radicle ; a diverging process of similar 

 character on each side, but not quite opposite ; a longitudinal axis of 

 greater or less extent ; and a sac-like covering, or thin pellicle of grapto- 

 litic test, which has scarcely assumed the form of cellules, but which is 

 most extended in the direction of the common body along the solid axis. 

 This little sac contains the germ of the zoophyte, which, extending itself 



*]In the recent Sertularia and Campanularia we find ovarian vesicles, in which a 

 number of ovules may be enclosed in a common envelope. These vesicles are developed 

 along the side of a stipe or branch, and the cvules are often arranged along a central axis, 

 each one communicating with the common axis of the zoophyte. [Jas. J. Lister, Philo- 

 sophical Transactions, 1834, pp. 365-388, pi. ix. Cited also by Dana, " Structure and 

 Classification of Zoophytes.''] 



Prof. M'CoY has stated (British PalcBOzoic Fossils, p. 4) thnt he has found near tho 

 base of the cellules of graptolites, a transverse partition or diaphragm, similar to what 

 may be observed in some sertularians, and which he regards :is ])roving similar relations; 

 but I have not discovered, in any American specimens, evidence of such cell -diaphragms. 



[Assem. No. 239.J 2G 



