their Structure and Affinities. Q6 



suited equally to both. As the terms, however, have not yet been 

 generally introduced into text books, it is perhaps desirable to give 

 definitions of those that I shall have to use in this paper. 



Every hydrozoon exists under two separate forms, the one the 

 " trophosome" destined for nutrition and growth, the other the " gono- 

 some " for re2)roduction. As nothing representing the gonosome of 

 graptolites lias yet been or is likely to be observed, we omit the 

 terms proposed in connection with it, and define only those applied 

 to the trophosome. The " coenosarc " is the common connecting basis 

 of the colony, organically uniting the various individuals or '^ poly- 

 pites." The soft parts are more or less completely protected by a 

 chitinous "polypary"; to that portion investing the coenosarc we 

 confine the term "periderm," and the cup-like receptacle in which 

 the individual polypite exists is called the " hydrotheca." The 

 ''hydrorhiza" is the root-like termination of the polypary by which 

 the compound hydroid is attached to foreign bodies, and the " hydro- 

 caulus " is the portion of the polypary that intervenes between the 

 hydrorhiza and the first hydrotheca. In the Sertulariadcd the " gono- 

 phores " or generative buds are produced from the sides of the '' gono- 

 blastidium," or column which passes up the centre of the "gonotheca," 

 or *' gonangium." ^ 



The general structure of the graptolite is very simple. The 

 polypary only has been preserved, and that consists of a periderm 

 sometimes forming a thread-like tube, from which the hydrothecae are 

 given off at regular intervals (Bastrites), or a larger tube, the one face 

 of which is formed of the conjoined bases of the uniserial hydro thecae 

 (Graptolithus, etc.). The back of the polypary is strengthened by a 

 slender solid axis, composed of the same substance as its walls, and 

 capable of separation from the rest of the polypary. The graptolites 

 with a double series of cells have a similar structure, being composed 

 as if of two single-celled forms united along the back of the periderm, 

 as is evident from the forms constituting the genus Dicranograptus, 

 in which a double polypary at the proximal end is more or less 

 speedily resolved by division into two branches, with a single series 

 of cells on their outer aspect. 



From this definition I exclude several forms which have been 

 referred to graptolites. First, two remarkable genera of double grap- 

 tolites representing perhaps different sub-orders, — EetioUtes, Barr., 

 which has no central axis, but the two series of cells rise on either 

 side of a single internal canal which occupies the central portion of 

 the polypary, and Phjllograptus, Hall, with a solid central axis, but 

 destitute of a common canal, the plates of the different cells being 

 continued to the solid axis. Also, the genera described by Hall and 

 Emmons, in which no cells have been detected ; and lastly, the reti- 

 culated fossils named Dictijonema, the affinities of which to grapto- 

 lites seem to me very doubtful. 



The structure I have described is that of the normal members of 

 the order. Before complicating our investigations regarding their 

 modern representatives, by introducing the consideration of the ab- 

 ^ Allmann in Ann. and Mag. of Nat. Hist., May 1864, p. 350. 



VOL. V. — NO. iLlV. 5 



