Br. J. J. Bigsby on the ' Thesaurus SiluricusJ 301 



I. These recurrents are mostly distinct from the intra-epochal, 

 owing to their first appearance being in the Upper Silurian stage. 



II. With the exception of Chonetes sarcinulattty they all stop 

 within the Devonian Period. 



III. The greater part of these recurrents are of low rank ; 20 

 are Brachiopoda ; 1 1 Zoophytes, 1 an Amorphozoa ; 7 are Gastero- 

 poda ; 3 Cephalopoda ; and 1 Trilobite. Manon deforme and Orthis 

 rugostty Lower Silurian fossils, reappear in Devonian, but not in 

 Upper Silurian, where they are " presumably " — to use an expression 

 of Mr. Etheridge. 



IV. These species are very migratory — few being found in two 

 epochs in the same country, but in different countries. 



V. Opportunities of escape mto a new epoch have been common ; 

 but the ways and means are frequently concealed by denudations 

 &c. 



VI. Acclimatization must have been necessary. 



VII. The length of individual life in proportion to specific extra- 

 epochal life is almost as a unit to infinity. 



Geographical Distribution of Silurian Life, 



The * Thesaurus ' tends to show that North America, east of the 

 Rocky Moimtains, may probably be divided into two areas, — the one 

 to the north of 57° (or of Lake Superior) being chiefly Upper Silurian, 

 resting on crystalline rocks, the one to the south of that line, down 

 to the Gulf of Mexico, on the contrary, being fully developed in 

 some part of this great space. 



It exhibits the regrettable fact that Asia, Africa, and Australia, 

 taken together, have hitherto yielded only 200 species of Silurian 

 remains ; but this arises from the absence of exploration. 



I have not yet had opportunity to bring together, harmonize, and 

 compare the Silurian life of the several countries of Europe. The 

 accomplishment of such a task might produce some definite truths, 

 and many more probabilities. Either this vast region would prove 

 to be one great Silurian area, with barriers here and there, and with 

 certain channels of communication, and to be the result of many 

 operations throughout a long interval of time ; or it might turn out 

 that the Silurian deposits and their fossils occupy three separate 

 areas: — (1) the Britanno-Scandinavian, which has all the three 

 stages, and the Primordial ; (2) the Bohemian, at present of peculiar 

 interest ; and (3) the middle and southern area, found in France, 

 Spain, and Sardinia, almost wholly Lower and Mid-Silurian. 



Under this head of geographical distribution we have to deal with 

 some curious phenomena — such as concern birthplace or first ap- 

 pearance, generic and specific, the duration of life, tolerance of con- 

 ditions, mineral habitats. Migration possesses great interest, with 

 its marks, causes, and modes, with its power, direction, and rate of 

 progress, &c. 



The transport or removal of dead organic matter from place to 

 place, the " remaniement*^ of French geologists, is an important 



Ann. §• Mag, N. Hist. Ser. 3. VoL xix. 22 



