134 Bibliographical Notices. 



Although a unity may pervade the Silurian system as a whole, 

 yet that the members comprising it may present a diversity is fully 

 shown by comparing the local faunas, in their relation to the epoch 

 at which each class has attained its development, in different countries. 

 Thus, " The Cephalopods have flourished in the Silurian seas of 

 N. America, Russia and Sweden, during the deposition of the lower 

 division, scarcely a trace of them being found in the equivalent strata 

 of England and Bohemia ; these mollusks, however, abound in the 

 upper division of these latter countries, becoming rare at the same 

 epoch in Russia and N. America. England and Bohemia present 

 another contrast in the Brachiopoda. In the former country this 

 class attained its maximum towards the base of the upper division 

 or Wenlock stage, while the Cephalopods abounded in the superior 

 Ludlow beds. The reverse has happened in Bohemia, where the 

 maximum of the Cephalopods is found in group E, and that of the 

 Brachiopods at a higher level in group F." In comparing, however, 

 the geognostical and palseontological characters, there is a general 

 resemblance between the two great divisions of the Silurian strata of 

 Bohemia and those of the typical district of England, France, Sweden, 

 Russia, and N. America. 



The Palceontological portion, which forms the chief part of the 

 volume, is full of most curious and instructive data, and is divided 

 into two parts, the first comprising the special study of the different 

 parts and elements of Trilobites, with an essay on their classification ; 

 the second contains a detailed description of the genera and species 

 found in Bohemia. 



The first portion is treated in a series of sections, of which the 

 following are the most important : — The form and principal parts of 

 the body of Trilobites (illustrated in plate 1) ; — the elements com- 

 posing the head (pis. 2 & 3), as the glabella, the sutures, the eyes 

 (pi. 3), their form and proportion, the cheeks or lateral lobes, the 

 hypostome and epistome ; — the elements of the thorax, as the form 

 of the thoracic segments and their articulation, the two types of the 

 pleura (illustrated in pis. 4-6), the number of segments, the power 

 of coiling up, and the value of the characters furnished by it ; — the 

 elements of the pygidium, its form, axis, lateral lobes, and contour ; — 

 nature and ornaments of the shell of Trilobites ; — metamorphosis and 

 mode of existence of Trilobites; — the distribution of them, both as re- 

 gards their vertical and horizontal diffusion, and which is graphically 

 illustrated in pis. 50 & 51. 



Among the protean forms of Trilobites, the Sao hirsuta appears 

 the most remarkable. First described in 1846, its numerous synonyms 

 would lead us to infer that it had passed through all the vacillations 

 of an older palseontological nomenclature, for it rejoices in more 

 names than would gladden the heart of a Spanish Hidalgo. It has 

 been assigned to no less than twelve genera and twenty -two species ; 

 but the minute researches of M. Barrande have clearly proved that 

 between the embryonic state, which is simply a flattened disk with a 

 body axis, and the head and thorax not distinct, to the adult form 

 with nineteen articulations, this Trilobite has passed through eighteen 



