260 Nicholson — On Graptolites. 



full growth and size, and the specimen, which I figured in the paper 

 I have alluded to,' appears really to have been an instance of mere 

 juxta-position. The mucro would seem to be the point of attachment, 

 and the organ whereby it is effected, as we might expect from 

 analogy ; but I have no certain evidence in support of this view. 

 The capsules apparently arise from no absolutely constant point of 

 origin, some springing from the common canal (Plate XI. Fig. 16), 

 others from the apex of a cellule (Plate XI. Figs. 14-15), and 

 others from the under-surface of a cellule (Plate XI. Figs. 13-14) ; 

 the last two modes being the most frequent. The largest capsule 

 which I have seen attached in this way has not measured more than 

 one-tenth of an inch in diameter, many being much smaller. After 

 attaining this size they seem to become detached ; numbers, often 

 nearly half an inch in length, occurring in this condition in the 

 shales, along with innumerable germs. The sequence of phenomena 

 thus described has not been observed by me in any other Graptolite 

 except G. Sedgwichii ; but it accords perfectly with the excellent 

 description and figures given by Hall of the reproductive process in 

 D. Whitfieldii? I have myself in many instances seen bodies essen- 

 tially similar to those described by Hall, though not so well preserved, 

 attached to the stipe of this same species from the Dumfriesshire 

 shales, and their occurrence appears to me to be inexplicable, except 

 on the hypothesis that they are reproductive. 



In certain other beds in the same locality (viz., Garple Linn), in 

 which G. Sagittarius is the prevailing form to the almost total ex- 

 clusion of G. Sedgwichii (Bastrites triangulatus) , and in which the 

 capsules occur in the greatest profusion, I have, nevertheless, failed 

 to detect any organic connexion between the two, I should be 

 inclined to explain this by the belief, that the gonophores of G. 

 Sagittarius were thi'own off when still extremely minute, subsequently 

 attaining their full developement. This view is borne out by the 

 occurrence of these bodies in all stages of growth, from small rounded 

 bodies, not bigger than a pin's head, up to nearly half an inch in 

 length, and it is further supported by analogy with many of our 

 recent Sydrozoa. I may observe, too, that in the rare instances in 

 which G. SedgwicJdi is found in this bed, the capsules are found 

 attached just as in the specimens from the beds, in which it is the 

 dominant species. If this view is not correct, it is still possible that 

 in the case of G. Sagittarius and, perhaps, other species, the gono- 

 phores were attached to the sides of the polypites, or to "gono- 

 blastidia," as in many living Hydrozoa. In this case the capsules 

 would, of course, never be found in organic connexion with the 

 Graptolites in a fossil condition. Another difficulty is presented by 

 the existence of beds like those of Hart Fell, where Graptolites are 

 very abundant, but where the capsules do not appear to occur at all. 

 Here, however, all, or almost all, the known species except Pleurograp- 

 sus linearis are referable to the genera Diplograpsus and Didymograpsus ; 

 whilst G. Sagittarius, G. Sedgwichii, and other common monoprionidian 



^ Op. cit. supra, Plate xvii. Fig. 3. 

 2 « Graptolites of the Quebec Group," p. 33. Plate £, Figs. 6-11. 



