Dr. H. A. Nicholson on the Graptolitidte. 06 



IX. — On the Nature and Zoological Position of the Graptoli- 

 tidse. By Henry Alleyne Nicholson, D.S*c.,M.B,,F.G.S. 



[Plate UI.] 

 The Graptolitidae constitute a group of extinct organisms 

 which may be considered characteristically Silurian, though 

 one genus {Dictyonema) passes up as high as the Middle Old 

 Red in America. Their zoological position has always been 

 a matter of doubt ; and they have been referred by different 

 palgeontologists to the Cephalopoda, the Hydrozoa, the Actino- 

 zoa, the Polyzoa, and recently to the Foraminifera. The first 

 and last of these views require no further notice ; but the re- 

 maining three are still maintained by different competent autho- 

 rities, and the question must be looked upon as still undecided. 

 In the following brief description of the morphology, develop- 

 ment, and reproduction of the Graptolitidfe I purpose to draw 

 attention to the facts which appear to favour the view, origi- 

 nally put forth by Prof. M'Coy, that the group should be re- 

 ferred to the Hydrozoa. 



Morphology. — As to the morphology of the Graptolitidae, 

 the simplest form of Graptolite is composed of three factors, 

 structurally and developmentally distinct, but united into a 

 single linear stipe. These three elements (PI. III. fig. 2) 

 are known as the " solid axis," the " common canal," and the 

 " cellules ; " and when combined together the solid axis is 

 found as a cylindrical filament, or laminar plate, having on 

 one side of it the common canal, from which, as a common con- 

 necting substance, arise the denticulated cellules. In this way 

 are formed those simple forms of Graptolites (PI. III. fig. 1) 

 to which alone the genus Graptolites or Graptolitlms ought to 

 be restricted ; and by the combination of such in various dif- 

 ferent modes are formed all the remaining generic types of the 

 Graptolitidge. The simple uniserrate Graptolites thus com- 

 posed, such as G. Sedgivicldij G. Sagittarius^ &c., have cer- 

 tainly no direct representatives amongst either the Hydrozoa 

 or the Polyzoa ; but the corneous nature of the entire polypary 

 and the presence of a " common canal " would seem to refer 

 them to the former, since the latter have, as a rule, a more or 

 less calcareous test, and the individuals forming the compound 

 organism are not united by any organized connecting substance. 

 There is, besides, an obvious resemblance between the mono- 

 prionidian stipes and the separate branchlets of some of the 

 Plumularise, such as Plumularia pennatida and P. cristata ; 

 whilst the diprionidian forms constituting the genus Di- 

 plograpsus have an equally obvious analogy to the ramuscles 

 of some ^~ - " ' ■ 

 S. flicnkf. 



