part 4] THE EVOLUTION OF LIASSIC GASTEOPODS. 309 



Procerithidie : the obliquity or straightness of the columella may 

 also serve broadly as a differential chai-acter, but it is of little 

 use for anything except the widest divisions. The soundest 

 classification, therefore, takes into account the following cha- 

 racters : — 



A. Differential. 



(1) The nature of the ornament that first appears, whether axial or spiral. 



(2) The character of the aperture, whether holostome or siphonostome 



(to be used with the reservation that most primitive forms are 

 holostome). 



(3) The obliquity or straightness of the columella. 



(4) The character of the embryonic whorls, whether heterostrophic or 



homoeostrophic. 



(5) The curve of the growth-lines. 



(6) The spacing of certain features of the ornament on the whorl. 



(7) Differential acceleration or retardation of characters. 



B. Progressive. 



(1) Shape of whorl. 



(2) The curve of the growth-line (or concrescent ornament). 



(3) The form of the aperture (in canaliculate species). 



(4) The size of the shell. 



(5) The strength and character of the ornamentation. 



(6) The contraction of the spiral angle towards the base. 



In the detailed treatment of genera and species these characters 

 are dealt with, and an attempt is made to show the relationships 

 of the species. This has been possible in certain cases, since 

 several series of specimens, presumed to represent actual lines of 

 descent, have been available ; but in no case have the later members 

 of such lineages sliown a very marked advance on the earlier 

 members, and it has not been easy to trace the groups from zone 

 to zone. 



For these reasons it has not been possible to use in a satisfactory 

 way many of the subgeneric names proposed by Dr. Cossmann, 

 neither has it been deemed advisable to add anv sreneric names at 

 the present time. When further material is available for study, 

 and relationships can be clearly established, more detailed classifi- 

 cation may be possible ; but, for the present, we propose to use 

 the existing generic names, it being understood that in certain 

 cases thev cover several lines of descent. 



IV. Family or the Peoceeithidj!: Cossmann. 



The fossils referred to this family are generally characterized by 

 fairly simple mouth-form, with both spiral and axial ornamenta- 

 tion. In most species the axial and spiral types of ornament are 

 approximately equal in strength, and give rise either to a lattice- 

 type of ornament, or to feeble tuberculations. In a few species 

 the axials are much more prominent; while in others, chiefly 

 owing to the feebleness of the axials, the spirals appear most 

 prominent. 



