58 Dr. H. A. Nicholson on the Nature and 



semble somewhat the " hydrothecas " of the Sertulariclse ; but 

 they are invariably in contact for a greater or less portion of 

 their length, and they do not seem ever to be cut off from the 

 common canal by any partition or diaphragm. 



Development. — The ordinary germs or embryonic forms 

 of Graptolites (PI. III. figs. 12, 13), in the youngest condition 

 in which they are preserved to us, are minute corneous bodies, 

 consisting of a small radicle or mucro, which is in fact the 

 commencement of the solid axis. This, the primitive structure 

 of the embryo, is widened out distally by the presence of the 

 common canal on one or both sides, according as the germ 

 belongs to a mono- or to a diprionidian species ; and finally 

 one or two primordial cellules are superadded. Even at this 

 stage the solid axis projects beyond the primitive cellules as a 

 naked rod ; and its growth was probably can-ied on to a certain 

 extent independently of the rest of the organism. These germs 

 are various in size, and differ in minor details ; but they all pos- 

 sessed a corneous envelope, and they cannot safely be compared 

 to any of the embryonic forms of the existing Hydrozoa 

 or Bryozoa. It must, in fact, be considered very probable 

 that these germs, as we see them, are considerably advanced in 

 growth, and that the earliest form of the embryo was devoid of 

 any corneous test. 



As to the further development of the stipe, it must suffice to 

 state that in the simpler genera the secondary cellules appear 

 to be intercalated between the initial point or radicle and the 

 primordial cellule or cellules, so that the youngest cellules are 

 proximal, the oldest distal in position. This mode of develop- 

 ment corresponds with that observed in the Calycophoridas and 

 Physophoridge amongst the Hydi'ozoa. 



Reproduction. — Until quite of late years, the reproductive 

 process in the Graptolitidee was quite unknown ; and even now 

 our knowledge is partial and to a great extent inferential. In 

 1858 Prof. Hall gave a description of some specimens oi Di- 

 jplograpsus Whitjieldii bearing bodies which he regarded as 

 ovarian vesicles. According to Hall, these appear as small 

 ovate buds attached to the stipe on both sides, enlarging to 

 form elongated sacs, and finally dehiscing. These sacs are 

 limited along their margins by a filiform border like the solid 

 axis of a Graptolite ; and it appears inconceivable that their 

 function can have been other than reproductive. 



At the meeting of the British Association in 1866, I 

 drew attention to the occurrence in the Graptolitic rocks of 

 Dumfriesshire of bodies essentially similar to those described 

 by Hall ; and I have since described them more fully and 

 have adduced instances in which they are seen attached to 



