Graptolites. 289 



to the aspect of Hall's remarkable genus, which he has named 

 Betiograptus, in which the connecting axis is prolonged from 

 the older portion of the organism. But the discovery of this 

 form by Hall suggested the possibility of the supposed perfect 

 specimens of Diplograpsus being only fragments of more com- 

 plex forms, and on laying this slab open my first impression 

 was that I had discovered such a perfect polypary. A con- 

 nection by means of the growing axis would be somewhat 

 anomalous, unless we supposed that the growth of the graptolite 

 was like that of those oceanic hydrozoa whose hydrosoma 

 consists of several polypites connected by a flexible filiform 

 ccenosarc, which grows at its newer portion, or where it is 

 attached to the swimming bell. But that this could not have 

 been the method of growth is shown by the branching and 

 rebranching form to which I gave the generic name of 

 Cladograpsus, the ultimate divisions of the polypary of which 

 has the axis produced beyond the growing polypiferous, and 

 necessarily free portion. Neither could there have been any 

 connection in Diplograpsus by the axis prolonged from the 

 proximal end of the organism, for innumerable specimens 

 show that it terminated at this end in one, two, or three spines 

 or processes. In D. scalarls, His., the termination is simple; 

 in D. bicomis, Hall, double ; and in D. tricornis, Car., triple. 

 The form and size of these processes can be distinctly seen, so 

 that the organism must be considered complete at this the 

 proximal end. 



The openings of the hydrothecas are, in some species, fur- 

 nished with one or more processes (Fig. 17), in some short and 

 firm, in others long and slender. 



Form. The general form of the graptolite was, first, that of 

 a slender, linear, indefinitely-produced polypary with a single 

 series of cells, either simple (Graptolithus, L.), twice branched 

 (Didymograpsus, M'Coy), symmetrically branched on either side 

 of a primary point (Tetragrapsus, Salter, etc.), or repeatedly 

 and irregularly branched (Cladograpsus, Car. not Gein.) ; or, 

 second, a shorter and broader polypary with two series of cells 

 (Diplograpsus, M'Coy), etc. 



A more complex form has been found in America, which is 

 considered by Hall as exhibiting the perfect polypary of the 

 genus Graptolithus. The organism (Fig. 3) orignates in a 

 short, barren process called the " radicle," which divides into 

 two branches, which again divide more or less frequently, the 

 portion of the polypary being without polypites until beyond 

 the place of the final branching. The barren bases of the 

 branches are united by a central disc composed of the same 

 substance as the graptolite, and consisting of two layers, which, 

 at least in the centre, are not united. Whether or not all the 

 VOL. xi. — no. iv. u 



