A REVIEW Or THE TRILOBITE3. 275 



ever, to this property when discussing the affinities of the trilobites. 

 In the absolute number of their body-segments, a considerable 

 difference is exhibited by different species. In fully developed forms, 

 omitting the still doubtful agnostidce, the number varies from five to 

 twenty-eight- Several palaeontologists — more especially Quenstedt 

 and Bubmeisteb — have placed great stress on the relative numbers 

 of these segments : making the character indeed, the basis of their 

 classifications.* That the character is one of considerable value, is 

 undoubtedly true ; but its application is beset with much difficulty, 

 since the able researches of Babbande have shewn that, in most, 

 if not in all species, the number of the rings, although constant in 

 the adult form, varies with the earlier age of the individual. He has 

 thus traced the metamorphoses of one species (Sao hirsutd) from its 

 embryonic condition with merely a head and caudal shield visible, 

 up to its full development, in which successive rings are added to both 

 thorax and abdomen, until, in the former alone, their number amounts 

 to seventeen. The adult form in this small species is frequently 

 under an inch in length. 



-A. The caudal shield, to which the term of pygidium is also applied, 

 consists, like the head-shield or buckler, of a single piece. This, 

 however, as shewn by its divisional markings, is evidently made up 

 of consobdated or united segments. With certain special exceptions, 

 we here recognize, as in the thorax, a middle portion — the caudal 

 axis, tail-rachis, &c; and sides, or pleura?. The segment lines in these 

 divisions are often strongly marked, but always undivided, unless at 

 the ends of the pleura?. In some species the pygidium is very small ; 

 in others well developed. The axis also is in some species continued 

 far down, or almost to the extremity of the shield ; whilst in others 

 it is extremely short. Occasionally the shield terminates in a point 

 or spine, or is furnished with various spine-like processes. The ends 

 too of the caudal pleura? are sometimes free, sometimes merged in a 

 continuous limb. According to Babbande, the more developed the 

 pygidium, the higher the developement of the animal — in substantia- 

 tion of which it is pointed out, that in the trilobitic forms of earliest 

 occurrence, this organ is comparatively small ; whilst in those of the 

 higher rocks, the contrary is generally the case. To this, however, 

 there are many exceptions ; witness, for example, the Ogygia Minne- 

 sotensis (Dikelocephalus of Owen) of the Potsdam sandstone on the 

 one hand, and the Harpes macrocephalus of the Devonian series on 

 the other. 



* .ioe the Appendix. 



