J. nOPKINSON ON BRITISH GRAPTOLITES. 163 



young graptolites (PI. viii., /. 30j, The recent Tubularta, a 

 Hydrozoon, is propagated in a somewhat similar manner, " the 

 capsules giving birth to progeny closely resembling the parent." 

 (Carruthers). 



Of the development of graptolites we have more certain know- 

 ledge. I have found young graptolites in all stages of growth, 

 from a mere microscopical point to a completely formed polypary 

 (PI. vui.,/. 22 — 25). I have never found them in connection 

 with the supposed gonophores, or gonothecje. In fact, where young 

 graptolites are most abundant, these organisms are rare ; and in the 

 only locality near Moffat where Dr. Nicholson's grapto- gonophores 

 are numerous, I do not remember seeing a single germ. Germs 

 or young graptolites are figured in the "Geological Magazine" 

 and in the "Intellectual Observer" for 1867; in the former, by 

 Dr. Nicholson ; in the latter, by Mr. Carruthers. They are also 

 figured in Hall's " Grapt. Quebec Group" {PL viii.,/. 27—29). 



Diplograpsus pristis appears first as a minute triangular body, 

 with a microscopically slender axis (PL vii., F. 2b). The hydro- 

 thec« are then developed along the free axis, which continues lo 

 grow faster than they are produced. Of Diplograpsus tricornis 

 Mr. Carruthers says : — 



" At the earliest stages the young specimens show all the cha- 

 racters of the adult. There is the solid axis continued above the 

 polypary, and the three spines at the proximal end. At first a thin 

 membrane is spread out between the spines and the slender axis at 

 the distal end. Next there appear indications of the hydrotheca?, 

 and these increase in number until the organism attains consider- 

 able size (PL viii., /. 26). The growth of other genera is some- 

 what similar. Young specimens of Didymograpsiis have been 

 found with merely a radicle and a single hydrotheca on each side. 

 In most cases the first-formed thec^ are smaller than those after- 

 wards formed, and do not increase in size." 



In Diplograpsus folium the oldest thecje continue small ; but the 

 young ones increase in extent by layers added to their apertures. 

 We find a somewhat similar mode of growth in Phyllograptus. 

 In Ccplialograpsus the thecaj appear to elongate towards the 

 proximal end, and after a short time do not increase in number, the 

 young specimens having generally as many hydrothecee as full 

 grown individuals. 



The development of many Hydrozca is similar to that of grap- 



