Bibliographical Notices. i^ 



do not occur in the lower, as Harpes, Bronteus, Pro'etus, Deiphon, 

 Phraffmoceras, Ascoceras, Trochoceras, and Cardiola, &c. We may 

 remark, however, that in the genera common to the two divisions, the 

 species present considerable modification of form. Thus, in Acidaspis, 

 the species belonging to the lower division have ten or more thoracic 

 segments, while those of the upper possess only nine. Other specific 

 modifications are presented in the genera Cheirurus and Ampyx, 

 which are also found in both divisions. 



Decided as is the line of demarcation between the two great divi- 

 sions, by the outburst and spreading over of a vast mass of trappean 

 matter at the termination of the lower division, yet a curious fact is 

 recorded by M. Barrande, in the local appearance or colony of fifty- 

 seven species belonging to the third fauna E, within the limits of the 

 second or lower fauna D. This colony of species, limited in extent, 

 appears therefore to have survived, either by migration or otherwise, 

 the causes which effected the extinction of the whole lower fauna 

 with which it is intercalated. 



Of the three successive Silurian faunas, defined by the distribution 

 of the genera of Trilobites, the lower or primordial one C, contains 

 only one genus, Agnostus, which passes into the second, and that 

 under diiferent specific forms. This group corresponds to the Olenus 

 schists of the Malverns, and the Lingula beds of Merionethshire, &c. 

 In England, according to Mr. Salter, " Agnostus is generally charac- 

 teristic, not of the first, but of the second zone or true Llandeilo 

 flags." In the second fauna D, the genera of Trilobites have attained 

 their maximum development ; some are peculiar to it, others have 

 reached their numerical specific maximum, and a third set have 

 their greatest development in the superior fauna. Associated with 

 these are numerous Cephalopods, Brachiopods, Pteropods and Ace- 

 phala, which have a very unequal geographical distribution. This 

 group appears to be the equivalent of the Llandeilo and Bala beds of 

 N. Wales. 



These two faunas may be regarded as only subsections of the 

 lower division, and this appears to be the opinion of Mr. Salter, who, 

 from an examination of the British localities, states, " It may perhaps 

 be necessary hereafter to modify the conclusions drawn by so able 

 and successful an observer as M. Barrande, as to the primordial and 

 isolated character of his earliest fossil group ; it may be a local, and 

 not a general phsenomenon." The third fauna E-H, forming th3 

 upper division and equivalent to the Upper Silurian, contains many 

 genera of the second fauna, but the assemblage of the specific forms 

 and their characters are very different. Here, however, the species 

 of Trilobites, and not the genera, attain their maximum. 



The upper limits of this fauna, or its relation to the Devonian, are 

 perhaps not distinctly defined, although M; Barrande states that Silu- 

 rian Trilobitic types range throughout, accompanied by Brachiopoda, 

 Gasteropoda, &c. In a previous memoir on the Brachiopoda, by 

 M. Barrande, we thought we had recognized many forms very ana- 

 logous to, if not identical with, Devonian species of other districts ; 

 this, however, is a subject for further inquiry. 



