Revieivs — The PalcBontogvaphical Society of London. 137 



and genera. He remarks that ''when once the doctrine of Evolution 

 is admitted in its entirety, the ideas attaching to such expressions 

 as ' genus ' and ' species ' lose somewhat of their definition. Never- 

 theless, they are necessities in classification ; and even if they do 

 not absolutely exist in nature, it becomes necessary as by a sort of 

 legal fiction to presume that they do exist for purposes of scientific 

 arrangement." 



The author speaks favourably of the trinomial system, but 

 considers it would be unwise at present to depart from the old 

 course, not, however, because the binomial system was the more 

 philosophic or the better. " We must jog on as well as we can with 

 the help of sub-genera and var. a and var. h, and this no doubt can 

 be done without any difficulty." In this way the number of species 

 is kept down, and the subject remains free from that complexity 

 which must attend an undue multiplication of names. Even sub- 

 genera, when introduced, are rightly kept within parentheses, as in 

 the case of Nerinma {Nerinella) gracilis, Nerirma (Ptygmatis) pisolitica, 

 Turritella (Mathilda) opalina, etc. The author further observes with 

 regard to the classification of the fossil Gasteropods, that the subject 

 must be treated from a conchological point of view : " we must fall 

 back upon the shell as the basis of classification of our fossils, and 

 accept such aid from zoology as is consistent with the material at 

 our disposal." 



With regard to palasontological horizons, the author remarks that 

 " From their uncertain and unequal distribution, and also from the 

 fact that Gasteropoda have been greatly influenced by the physical 

 conditions that obtained during the period of deposition, it is pro- 

 bable that they are of less value as indicators of horizons than 

 the Cephalopoda, the Ammonites especially. Moreover, with the 

 exception of one or two groups, such as Nerincea, their mutations 

 have been much less rapid, so that forms have been more enduring, 

 and thus sundry demoid common types may be said to pervade 

 nearly all the beds." 



The author proceeds to give us an excellent account of the strati- 

 graphical features of the Inferior Oolite in its range from Dorset- 

 shire to Yorkshire. He has himself studied the rocks and their 

 fossils at most of the important localities, and has, moreover, 

 employed a fossil-collector to make diligent search, so that he has 

 accurate knowledge of the horizons of nearly all the species that 

 have come under bis notice. 



The Inferior Oolite is divided into two broad divisions as 



follows : — 



Zones. 



TT T^• • • f Ammonites Parkinsoni. 



Upper Dmsion ( ^^ Humphriesianus. 



i,, concavus (subzone). 



,, Murehtsonce. 



,, opalinus. 



Other local zones and fossil-beds are duly indicated in the detailed 

 accounts of various sections, so that this introductory essay on 



