30 



THE GEOLOGIST. 



showing the very slender organic connection between the deposits 

 under notice and those of the Silurian age. A glance at the Table 

 shows that, of the two, Petherwin is the nearest to the Lower Devo- 

 nian horizon, and the most remote from the Carboniferous; true, the 

 majority in each case is but small — 208 to 171, and 211 to 181 — but 

 it must be remembered that great ones were not expected ; and that, 

 feeble as they are individually, there is strength in the fact that their 

 testimonies agree ; if they mean anything, it is that the Barnstaple 

 beds are somewhat more modern than those of Petherwin ; a conclu- 

 sion to which more than one eminent geologist has been led by other, 

 and, perhaps, more reliable evidence. 



The fossils of the two areas belong to forty-six genera, of which 

 thirty-three are represented by the Petherwin, and thirty-four by the 

 Barnstaple series, twenty-one are common to both ; hence twelve are 

 peculiar to Petherwin, and thirteen to Barnstaple. The South Devon 

 and contemporary beds contain sixty-four genera, of which thirteen 

 only occur in the deposits now under notice. 



Taken as a whole, the forty-six genera above mentioned have a 

 Carboniferous, rather than a ^^lurian, or even a Lower Devonian 

 fades. They may be divided into^groups, namely, 1st, those characte- 

 rized by a considerable maximum specific variety or development in 

 some one period before or after Petherwin and Barnstaple times, that 

 is, during the Silurian or Lower Devonian eras on the one side, or 

 the Carboniferous on the other ; 2nd, those that are not thus dis- 

 tinguished. For example, the rich genus Ortlioceras had, in Britain, 

 an almost equal number of species in Carboniferous and Upper 

 Silurian times, when it was richest ; hence it had no one period of 

 maximum specific variety, and consequently belongs to the second of 

 the groups just defined; as, of course, do also all other genera simi- 

 larly characterized, as well as those, such as Hallia, which seems 

 never to have had more than a very few species at any one time. 



The first of these groups — which alone we have to consider here — 

 contains thirty-one genera, of which six may be said to belong to tlie 

 Fast, and twenty-five to the Future, the age of Petherwin and Barn- 

 staple being the chronological stand-point. 



The first, or Past " division, does not contain a number suffici- 

 ently great to be of service in this inquiry. The last, or " Puture," 

 consists of two series, namely, 1st, those genera which are equally 

 represented in the two sets of beds ; and 2ndly, those that are not ; 

 evidently the last series alone can supph' information on the question 

 under consideration. It is made up of the fifteen genera named in the 

 following table, in which the columns headed P., B., C, exhibit the 

 number of species, belonging to each genus, which occur in the 

 Petherwin, Barnstaple, and British Carboniferous beds respectively. 



Prom the table we learn that nine of these genera are found in 

 Barnstaple only, or are more largely represented there than in Pe- 

 therwin ; and that nineteen species represent the ten genera found 

 in the former area, and no more than ten the six genera of the 

 latter. Hence, the genera tell us what the species had told us be- 



