66 W. Carruthers — On the British Graptolites, 



normal genera, we must first determine the relations of these abnor- 

 mal structures to the type structure of the order, which has not yet 

 been done. 



I am induced to prefix to this revision of the British species a 

 statement of the evidence which induces me to place the graptolites 

 among the Hydrozoa at greater length, because of a paper recently 

 published by Dr. Nicholson on this subject (Ann. and Mag. of Nat. 

 Hist., Jan. 1868), which I will not characterise, leaving that to my 

 readers after they have become acquainted with some extracts that 

 I shall require to make from it. Every palseontological student will 

 allow that it is wasting time to imagine the purpose of obscure and 

 anomalous structures, or to discover for them by wild stretches of 

 the imagination fancied analogies with structures in recent tribes to 

 which the writer is anxious to ally them. There must be a cor- 

 respondence of some kind between the organisms better than a 

 " possibly," a " probably," or even an " apparently." If any trust- 

 worthy determination is to be reached, it must not be built upon 

 supposed resemblances about which there must always be a difference 

 of opinion, but upon structural points existing in the fossils that 

 are identical with, or at least similar to, what is found in recent 

 organisms. 



All evidence from the coenosarc and its appendages is necessarily 

 wanting in these palaeozoic fossils, and our comparison must be 

 based entirely on the polypary. 



Accepting the opinion almost universally entertained, that the 

 cells of the graptolite contained animals living in colonies, the 

 investigation as to their modern allies is limited to the coelenterate 

 and molluscoid zoophytes. The general form of the polypary is not 

 of importance, as on the one hand it is one of the most variable 

 characters in the same group, and on the other, it is one that repeats 

 itself in very different groups. It would be impossible to distinguish 

 between the Hydrozoa and the Polyzoa from general form ; besides, 

 this is very variable amongst the graptolites themselves. Neither 

 is the notion that the polyparies were free (granting that they were 

 so) of much significance, inasmuch as there are free forms among 

 both the Polyzoa and Hydrozoa. Nor can much stress be laid on 

 the chitinoiis nature of the polypary, for though that of the Hydrozoa 

 is always chitinous, in " many Polyzoa it is horny and flexible " 

 (Busk), and all the species of the order of Polyzoa which most nearly 

 resemble the graptolites have such a horny polypary. 



While the great systematic distinction between the Hydrozoa and 

 the Polyzoa depends entirely upon the remarkable difference of the 

 polypites, yet there has been found associated with each class so 

 peculiar a structure of polypary that there is no difficulty in deter- 

 mining to which group a particular polypary belongs. This struc- 

 ture is connected with the presence or absence of a common canal, 

 and with the relations of the cells which contain the polypites to it. 

 The true affinity of the graptolite will be easily determined, when 

 we ascertain to which of these two types of structure it most nearly 

 approaches. 



