162 J. HOPKINSON ON BRITISH GRAPTOLITES. 



both graptolites and recent Hyclrozoa, warrants us, I think, in 

 including them in this class, and it only remains for us to decide 

 their ordinal position. 



Professor Huxley divides the Hydrozoa into six orders, or, in- 

 cluding the provisional order, Medusidaj, seven. Of these, we may 

 dismiss from our consideration all but two — the Corynida3 and the 

 Sertulariadse — they only having a cuticular ectoderm. In the 

 Corynidge the hydrosome is developed into a ctenosarc, supporting 

 many polypites without thecfe ; in graptolites, the polypites were 

 contained in true thecae ; the genus Climacograptus is apparently an 

 exception. In the Sertulariadee, the ectoderm of the cffiuosarc 

 supports polypites enveloped in thecce. This is the structure of 

 the graptolite. 



But in neither the Coryuidse nor the Sertulariadae, nor, in fact, 

 in any Hydrozoon, is there a solid axis. "Were it not for this, we 

 might place graptolites in the order Sertulariadce ; but as it is, 

 we must consider them a distinct order, their precise zoological 

 position being between the Corynid^e and the Sertularidse, and 

 their nearest alliance being with the latter order. 



I should not omit to mention that I am indebted to Mr. Car- 

 ruthers for most of the arguments I have used. In his paper in 

 the " Geological Magazine" for the present year (1868), to which I 

 refer you for further information, this question is treated much more 

 fully than I have here attempted. 



§ V. Reproduction and Development. 



Of the mode of reproduction of graptolites little certain is known. 

 In the graptolitic shales of Dumfriesshire there occur certain oval 

 bodies bearing some resemblance to the gonothec£e of Sertularians. 

 They have been figured in the " Geological Magazine " for 1867, 

 by Dr. Nicholson, as actually growing out of specimens of Grap- 

 tolithus Sedgwickii ; but it is supposed that such appearances are 

 merely due to cases of accidental juxta2:iosition. In the autumn of 

 1866, when I first became acquainted with graptolites, I found 

 these organisms in abundance, in the precise locality from which Dr. 

 Nicholson obtained his specimens — Garple Linn, Moffat ; but I 

 could never trace their connection with any graptolite. 



In the " Graptolites of the Quebec Group " (of Canada), Hall 

 figures a Diplograpsus with reproductive sacs attached to its 

 periderm. Adjoining the margin of one of these sacs are two 



