20 Reviews — Bigshys Thesaurus Siluricus. 



But every such estimate must be of tlie rudest kind possible with 

 reference to a coal-field, in which not a single pit has yet been 

 sunk, nor a single opening made. — H. B.W. 



I^EVIE^WS. 



I. — Thesaurus Siluricus — The Flora anb Fauna of the 

 Silurian Period. By J. J, Bigsby, M.D., F.G.S., etc., etc. 

 London, 1868. Van Yoorst. 4to. pp. 268. [Second Notice.] 

 ( Continued from the Novetnber Ntmiber, p. 521.^ 



IN his introduction, Dr. Bigsby gives an analysis of the classes Gasteropoda and 

 Echinodermata, and the orders Trilobita and Brachiopoda as ^:hey -occur, both 

 in stratigraphical and geographical distribution ; — space, however, entirely forbids 

 our attempting any fresh analysis for tliis notice, the subjects being so prolific. 

 Under the head Gasteropoda in the introduction, pp. vii.-x., several tables are 

 constructed ; that on p. vii. gives the geographical summary of species for the 

 chief areas in whichSilurian life is known ; — 27 areas are given — 12 on the American 

 Continent, 3 in Britain (England, Ireland, Wales), 9 European, i South Australian, 

 I Tasmanian, and i for North India; but in the chief and detailed summaiy, p. 169, 

 m the body of the Thesaurus, 43 areas are given— 25 American, 15 European, 

 with India, Australia, and Tasmania in addition, and an analysis of all the genera 

 (51) and 895 species, with their appearances, numbers, and habitations. The table 

 (B) also containing those genera having the greatest range in time and space, with 

 the number of species in each and number of countries inhabited — :show a kind of 

 analysis that may be carried to any extent. The result of one line for the Pleuro- 

 to7>iar ice is, itself, suggestive. This genus contains 171 species, and they are dis- 

 tributed over 34 large countries \the fossiliferoics Silurian area of Canada alone is 

 70,000 square miles']. This dispersion or distribution applies to aU the 5 1 genera, 

 and species belonging to the class Gastei'opoda, the ratio of the proportion of species 

 in each genus, and the different degrees of distribution being due to life habits, 

 associated faunse, sea-bottom, temperature, locality, etc. The Thesaurus tells 

 us that 18 genera out of the 51 are known only in, and each confined to one 

 district, and 12 of these genera possess only one known species in each, and 7 

 have 2 species only ; supposing the genera to be well established, we only require 

 further research, probably, to discover many other species. We have, however, 

 much doubt as to the value of such genera as Cpdyptrcea, Cerithium, Delphimda, 

 Littorifia ? Naticella, Phasianella, Roiella, Siphonaria, occurring in the Silurian 

 rocks, until we certainly know more of them, and it is amongst such that they^zw 

 forms, above named, seem to occur. We mention this to guard against drawing 

 conclusions hastily upon doubtful determinations, as per centage in generic life is 

 of importance on either side, and leads to false conclusions upon the question of 

 first appearance, locality, etc., as propounded in the 7 doctrines laid down in the in- 

 troduction to the Thesauras. Particulars are given, in the body of the great table, 

 of no less than 894 species of Gasteropoda. The increment and decrement of the 

 Gastei'opoda in time, as exhibited over large areas through their physical history 

 and mutations, is one of the many problems which the Palseontologist is con- 

 stantly called upon to investigate, and thus to account for the " rise and decline," 

 "first appearance," and " extinction " of any group. Dr. Bigsby, at p. ix. (table 

 C), points out in 8 subdivisions of the Silurian system, the rise, culminating point, 

 and decline of the Silurian Gasteropoda ; — their extinction, like that of other 

 groups, which really took place at the close of the Silurian and commencement 

 of the Old Red Sandstone, in Britain, is a problem geologists have yet to solve. 

 The table D, on p. ix., is too important to be passed over ; it is, as the author 

 terms it, a "time table" of all the Silurian Gasteropoda, showing the number of 

 species in each genus through the four Silurian stages, and if the species in the 

 w'ork may all be relied on, opens up a mode of time and space analysis whereby 

 any known area, having one or all the stages, may be "censused." That this 

 early epoch in the Earth's history should have contained so great an assemblage 



