STUDY OP THE GRAPTOLITES. 173 



proportion of the species and genera appear never to have been attached 

 to the sea-bottom. 



It may not be easy to determine precisely the family to which these 

 graptolitic forms should be referred ; nor is it certain that the extensive 

 series now presented can all properly be referred to a single family. Gen. 

 PoRTLOCK has suggested that these bodies may constitute " several genera 

 belonging even to more than one order."* That they are true Polypi, 

 belonging to the Hydrozoa, I believe we shall be able to show, both from 

 analogies already established by various authors, and also from their mode 

 of development or reproduction as exhibited in some of the species. 



The specimens which have usually been observed or represented are 

 simple disconnected stipes, doubtless the dismembered or fragmentary 

 portions of fronds, which, presenting in the different species great varieties 

 of form and aspect when entire, are nevertheless composed of parts so 

 similar that these fragments, though indicating specific differences, offer 

 little clue to a knowledge of the entire form. 



Taking these species which, in the form of their cellules and in the 

 separated fragments of the frond, would be referred to Graptolithus proper, 

 and tracing them, as we are now able to do in many species, to their perfect 

 condition, we find a great variety of form and mode of growth. In the 

 simplest of these, we have two stipes diverging from a radicle or initial 

 point ; and the parts remain so complete as to admit of no doubt that this 

 is the entire skeleton of the animal. The cellules near the base of the 

 stipe are not so fully developed, while also those near the extremities 

 have not reached their full dimensions ; and the last one is sometimes 

 barely perceptible, or just assuming its form from the common body. These 

 characteristics are perceptible in the figure upon Plate iii. 



Fig. 2. 



Geaptolithus pennatulus. 

 In the next stage we have four simple stipes diverging from an initial 

 point, and all evidently entire, as shown in the development of the cellules. 



Fig. 3. 



Graptolithus brtonoides. 



Geological Report on Londonderry, Ac, p. 318. 



