Distribution of the Hhabdophora. 23 



Pal^eontological (Table X.) . — We now enter upon the 

 zoological or palaeontological department of our subject. The 

 Table given above, which has been drawn up from a careful 

 examination and comparison of the data already brought 

 forward, summarizes the results of our present knowledge 

 with respect to the vertical range of the known families, 

 genera, and species of Rhabdophora. It will be advisable to 

 review these results in order, for each family and species in 

 its turn, that the provisional conclusions we advocate may be 

 contrasted by the student with the speculations of former 

 investigators and with the results destined to be developed by 

 future discovery. 



Family i. Monograptidse. 



Contrary to the opinions of the earlier paleontologists, it is 

 now clearly apparent that the important family of the Mono- 

 graptidas is strictly confined to Murchison's Upper Silurian 

 system. With the exception of the abnormal genus Azygo- 

 graphy (Nich. & Lapw.) no unilateral genus of Graptolite 

 has hitherto been detected in strata of older date than the 

 Lower Llandovery. As we have already shown, the numerous 

 specimens of Graptolithus [Monograptus) Sagittarius, His., 

 M. tenuis, Portlk., and M. Nilssoni, Barr., noted by Hall, 

 Salter, Baily, and others from Arenig, Bala, and Llandeilo 

 strata were merely fragments of compound species. Where 

 these palaeontologists have given figures of the so-called 

 simple forms, a glance at the illustrative drawings is generally 

 sufficient to satisfy the graptolithologist that they are fractions 

 of compound forms. In other cases, the known presence of 

 bilateral genera, whose broken branches have a superficial 

 resemblance to the unilateral species, in great abundance in 

 the formations from which these supposed simple forms are 

 derived furnishes us with a very natural explanation of these 

 erroneous identifications. It is not impossible that future 

 research will lead to the detection, in the highest zones of the 

 Bala formation, of the forerunners of the prolific Monograp- 

 tidse of the Valentian and Salopian rocks. But, in the actual 

 state of our knowledge, the mere presence of a single species 

 of the Monograptidai may at once be set down as conclusive evi- 

 dence of the (Vjper) Silurian age of its containing beds. 



The range of the entire family appears to be coextensive 

 with that of the Silurian system proper. Its earliest forms 

 struggle into visible existence near the base of the Lower 

 Llandovery, along an horizon which, in the attenuated 

 Graptolite-bearing deposits of Scotland and Scandinavia, is 

 removed but a few feet from the summit of the underlying 



