530 Reviews — The Fauna and Flora of the Silurian Period. 



life in reference to vertical range or recurrency, as known in 1865. 

 In table x., p. xlv., the entire animal kingdom then known fossil 

 (5,184 species) in the Silurian strata is analyzed ; the number of 

 species in each stage or division of the Silurian system or species 

 typical of one horizon (Silurian) is given, the recurrent number is 

 then shown for all the groups, and the average given for each group. 

 To show the importance of the Thesaurus in the analysis of such 

 facts, we may mention amongst others that throughout the sub- 

 kingdom Ccclenterata, numbering 311 species, no less than 70 (or 

 18 per cent.) occur in a recurrent state, in the proportion of 51 for 

 the Lower Silurian, 14 for the Middle, and 5 for the Upper ; the Trilo- 

 hita, possessing 845 species, in the proportion of 91, 16, 32=139, or 

 14 per cent, of the whole order ; and so on through the animal king- 

 dom, every individual species can be traced in the body of the work ; 

 and believing in the fidelity and trustworthiness of the author, and the 

 pains he has taken to arrive at truth and exactness, we may assume 

 that the facts and their deductions are reliable. The universality of 

 recurrence in marine life, whether ancient or modern, is traceable, 

 and common to all and every part of time, — every succeeding epoch, 

 from the earliest upwards, exhibiting this. It may be, as the author • 

 states, a measure of vitality or capacity for enduring change of food, 

 pressure, temperature, etc., the number of recurrents being a mea- 

 sure of new conditions, and the conditions favouring or rendering 

 this possible are simplicity of structure, fecundity in reproduction, 

 longevity, power of locomotion, facility in transportation, etc. etc. 

 Whilst sediments (many of which recurrents tolerate) are slowly 

 accumulating, generations mount up with the increasing thickness. 

 Entire areas of the world show these conditions exemplified through 

 the Trilohita, Gasteropoda, Cephalopoda, and Coclenterata of the 

 Silurian stages. So largely is this question dealt with in generalities 

 and suggestions in the Thesaurus that we must refer the reader to the 

 section, and tables X and Y, pp. xlv. and xlvii. 



7. Divergence. — The author of Thesaurus has constructed a table 

 at page 49, which he terms " Molluscan sea-grounds," ranging over 

 eleven large regions. It is done with the view of arriving at some 

 definite conclusions as to the nature of the sea-bottom, its sediments, 

 and feeding- grounds for the mollusca, etc., etc. 



This table may be applied to the same or similar conditions 

 through all Silurian time ; eleven kinds of sea-bottom are also 

 registered, which may be assumed as habitats, and over which the 

 dredge has passed. Forbes investigated the sea-grounds, etc. of 

 North-west Scotland, South and West England, and the ^Egean Sea ; 

 J. Gwyn Jeffreys, the British seas generally ; McAndrews, the North- 

 east Atlantic, Vigo, and Carthagena Bays, the Mediterranean, and 

 Norway ; Gumming, the East and West Pacific ; C. B. Adams, 

 Panama and South America ; and Hinds, the West Pacific. 



In 1863 3138 species had been dredged, and in these there appear 

 to be 1993 divergent species. The last four columns in the table 

 may now be considerably added to, but the principle is the same. 

 The result of 5000 acts of dredging are recorded by these ex- 



