May ii, 1882] 



NATURE 



27 



equivalent to M' Coy's sub-genus Temnocheilus ; 4, the 

 Siimesi, those with sinuous sutures ; and 5, those species 

 with nummuloid siphuncles (Trias only). 



Probably only three species of Nautilus occur in the 

 Silurian rocks — frautilus guadra/ts, N. Holtianus, and 

 N. Bohemicus. This last-named species appears to be 

 the Lituites Biddulphi of Sowerby (1838) ; but his insuf- 

 ficient description, in the " Sil. Syst." of Murchison, pre- 

 vents true identification. They are all three Upper 

 Silurian forms. The Lower Silurian rocks of Newfound- 

 land and Canada have yielded eleven species. In the 

 Carboniferous strata Nautilus attains through Discites its 

 maximum development. In the Permian only one species 

 is British, and three American, and is now the only living 

 representative of the Tetrabranchiata. The sub-genus 

 Trocholites (three species are Lower Silurian. These six 

 forms are the only true Nautiloidea in the Silurian rocks. 



The last group (4, the Irregulares) in Mr. Blake's 

 classification receives three genera : Trochoceras, with 12 

 species ; Lituites, 2 ; and Opliidioceras, 2 species. We 

 have no representative of either the Endogastric or Exo- 

 qastric group of Barraude in this country. American and 

 Swedish Lower Silurian species are somewhat abundant, 

 in Britain it is chiefly an Upper Silurian genus. Of the 

 12 species known 8 are Upper Silurian — Wenlock and 

 Ludlow — and the Llandilo, Bala, and Llandovery beds 

 yield the remaining Trochoccri. Five of the 12 species 

 are new to Britain, described and figured by Prof. Blake 

 for the first time. The two type species — Trochoceras 

 (Lituites) comu-arietes (Sowerby), and Trochoceras (Litu- 

 ites) giganteum (Sowerby) — are admirably refigured and 

 described by Mr. Blake, the general descriptions adding 

 to their specific value, and the determination of Bohemian 

 forms of Lituites in Britain materially adds to the corre- 

 lation of the two faunas. Barrande's genus Ophidioceras 

 {Lituites, ai/ct.) seems to have been recognised by Mr. 

 Blake ; it differs from typical Lituites by the whorls being 

 accurately in contact. The Lituites articulation, Sow., 

 was long ago figured as Lituites in the " Sil. System," 

 p. 622, t. 1 1, f. 5, and adopted by Salter. The straight 

 ribbing and band distinguishes this genus from Lituites. 

 This volume concludes with general observations, and 

 highly suggestive many of them are. Prof. Blake en- 

 deavours to throw some light upon the laws which govern 

 the appearance and disappearance of forms of life, and 

 into the nature of those groups of individuals to which we 

 assign the term species. 



Mr. Blake prepares a table, condensed from the larger 

 and preceding one on pp. 233-236. This condensed table 

 shows the numerical value of the species occurring in the 

 II genera, and ranging through the ten formations or 

 horizons, thus showing their increment, decrement, and 

 stratigraphical distribution. Both tables show three 

 maxima in the Ludlow, Wenlock, and Bala beds — in the 

 Ludlow 65 species, the Wenlock 43, and the Bala 39. 

 Mr. Blake does not believe that there was a correspond- 

 ing falling off between these epochs ; he draws conclu- 

 sions from the comparative fewness of species in the 

 Wenlock Limestone as compared with the Ludlow shales 

 above and Wenlock shales below that formation. Of 

 the four groups given the Cont'd first appear, and con- 

 : stitute the bulk of the Lower Silurian fauna, 31 species 

 occurring in the Bala rocks [the Tremadoc only yielding 



two species, the Llandeilo 9, and the Lower Llandovery 

 8, or 19 for the three horizons]. The Cont'd and Spirales 

 are the only two groups which continue on in time or 

 range into the higher divisions of the Pateozoic rocks 

 (the Devonian and Carboniferous'), the Inflati being re- 

 presented by Polerioceras, &c, in the Carboniferous series. 



Mr. Blake next considers the characters of the indi- 

 vidual genera and their appearance in time, but somewl.it 

 begs the question to suit his particular view upon evo'u- 

 tion ; it surely can hardly be safe to speculate upon any 

 particular curve or part of the curve in any particular 

 genus, to argue for descent through evolution, other con- 

 ditions not known. Neither Cyrtoceras precox or Orthoceras 

 sericeum — which are the oldest species known in Britain 

 — "are transitional forms, both being well characterised, ' 

 and it is questionable whether the group which has been 

 longest in existence in a given area, will there most 

 abound, many physical conditions may tend to prevent 

 that, " though we grant that possibly the greater the 

 abundance of individuals, the greater is their chance of 

 preservation in the rocks, the nature of the deposit ad- 

 mitted. 



In the paragraph on p. 239, having reference to the 

 genera Cyrtoceras, Phragmoceras, and Gomphoceras, 

 more evidence is wanting before we can draw any conclu- 

 sion as to priority of appearance, or show that tho=e 

 having the siphuncle internal (Endogastrica) appeared first 

 or preceded the Exogastrica (with the siphuncle external) ; 

 it is true we must take the evidence as it stands, or as we 

 find it ; it is, however, wiser not to theorise upon such 

 slender materials. 



Prof. Blake next notices and discusses the question of 

 species (pp. 239-243), and has pronounced opinions 

 upon this vexed, complicated, and philosophical question, 

 naturally the old idea of the independence of species is 

 rejected. Mr. Blake adopts all through bis book the 

 method of " actually describing a type-specimen around 

 which the other forms designated by the same name 

 may cluster.'' When the original type has not been seen 

 or found, a type is selected to which others are compared. 

 In attempting the explanation of the phenomenon of 

 distinct species or specific groups, it is well known that 

 two theories are now held : (1) that which considers each spe- 

 cies a "special creation, though inexplicable" ; or,(2) "that 

 which asserts the development of one from the other by a 

 process of evolution." Mr. Blake appeals to palaeontology 

 to show, through its researches, the gradations between 

 one species and another. The result to Mr. Blake's in- 

 vestigation in this and other groups is against " fixity ot 

 species." He states, that "if species were such definite 

 entities as they were once supposed to be, they ought to 

 be much more easily distinguished than they are, and that 

 the many variations of form which will be found included, 

 and necessarily included under one specific title, whose 

 'general description ' thereby becomes one of consider- 

 able latitude, show that different specimens are not so 

 closely linked as that theory would imply." On the other 

 hand, " Does this present study," asks Mr. Blake, " give 

 any positive aid to the theory of evolution ?" He 'ails 

 to see any reasonable solution or answer. It is evident 

 that among "the many forms which flourish in any one 

 epoch, it must be impossible to say with certainty which 

 was the descendant of any particular form in the pieced- 



