C. Lapivorth — O71 British Graptolites. 501 



in a work of detail. All wLa are likely to be interested in these 

 conclusions will notice at a glance what is original, and what 

 has been derived from the discoveries and speculations of previous 

 observers. 



DEVELOPMENT. 



A careful study and comparison of great numbers of young Grap- 

 tolites from the Moffat Shales, belonging to the genera Dicellograptus, 

 Dieranograptus, Pleurograptus, Leptograptus, Diplograptus, etc., has 

 induced me to conclude that in all the bilateral and duplex species 

 the mode of development is essentially the same. The polypary of 

 every species which I have been able to trace satisfactorily through 

 the earlier stages of its existence, first becomes visible as a small 

 pointed, triangular — or rather dagger-like — " germ," which from its 

 constant and characteristic shape I have denominated the sicula} In 

 the majority of the species examined the sicula is at first exceedingly 

 minute, but it sometimes attains a length of one-fourth of an inch 

 before it undergoes any perceptible alteration of its extex'nal aspect. 

 The first visible change consists in the development of a solid axis 

 in its outer wall, extending along its entire length, and prolonged as 

 a naked filiform process beyond one or both of its extremities. 

 A small protuberance or bud ^ next makes its appearance upon one of 

 the lateral margins, usually in the immediate neighbourhood of the 

 major extremity of the sicula, and this is gradually moulded into a 

 theca. A similar bud is then given off from the opposite margin, 

 but slightly in advance of the former, and from these primordial buds 

 the two main branches of the polypary are slowly evolved by a 

 process of continuous gemmation. 



- The mode of growth here described certainly obtains in most of 

 the species appertaining to the genera named above. In Ccenograptus 

 and Nemagraptus, however, the primordial buds are given off midway 

 between the two extremities of the sicula. In Dichograptus and its 

 allies it is yet uncertain whether the remarkable non-polypiferous 

 " funicle " peculiar to this group is formed from true buds, or 

 whether it is not rather a dichotomising outgrowth of the major 

 extremity of the sicula. The sicula itself normally ceases to grow- 

 after the primordial buds have originated. In rare examples of 

 Diplograptus it seems to be imbedded in the proximal portion of the 

 polypary. In a few dibrachiate species it is absorbed or becomes 

 obsolete in old age; but in the great majority of cases it permanently 

 retains the shape it possessed at the period of the commencement of the 

 lateral branches, and forms a very conspicuous feature in the adult 

 organism. It is simply this persistent sicula which constitutes the 

 " axillary spine " in 'Dicellograptus, the " radicular bar " in Coeno- 

 graptus, and the " radicle " in Didymograptus and Phyllograptus, etc. 



In some cases, however, the sicula most certainly increases in size 

 after the branches have been thrown off. In the genus Leptograptus 

 and its nearest allies it is frequently developed into a third and 



^ The sicula is probably identical with that portion of the polypary which Herr 

 Richter has termed the ^'foot." See Appendix. 

 * See Appendix. 



