CLASSIFICATION OF THE PRIMORDIA 113 



a series of successive sections from the summit towards the 

 base it would be possible to construct the developmental curve 

 of each primordium, 



A number of subjects of research may be found along this 

 line of investigation. 



SECOND EXAMPLE : The siphon of the Pelecypods. 



THIRD EXAMPLE : The shell of the Molluscs. This shell 

 consists of two or several superposed layers. Each layer may 

 he looked upon as being a chess-board system, the axes of which 

 are periclinal and anticlinal. In each layer (system) aU possible 

 alterations (§ 79) may exist and are combined in various ways. 

 The successive layers are superposed according to an axis ZN 

 (normal to the surface of the shell). They are more or less 

 independent of each other. It happens, for instance, that in 

 the external system (superficial layer) the segmentation in the 

 direction of the anticlinal axis^ is predominant, whereas the 

 segmentation according to the periclinal axis (division into 

 spirally curved resp. radiair segments) prevails in the second layer. 

 In other cases the same segmentation (anticlinal or pericHnal) 

 is equally predominant in both layers. In certain species the 

 second layer is completely concealed by the exterior one, the 

 properties of the latter being visible at the surface of the shell. 

 In other species the exterior layer is moulded upon the second 

 layer in such a way that the former bears the impression of 

 certain properties of the latter, the external characteristics of the 

 shell being a combination of the properties of both layers. In 

 many shells the coloured lines, spots, etc., are vexillary marks 

 (see § 81). In certain species oblique axes are observed (see 

 § 86, p. 109). Very often segmentation is obsolete, except at 

 or near the margin, where it is indicated by teeth (Donax 

 anatina, etc.), coloured spots (species of Cassis, etc.), etc. 



All the mentioned properties may be studied and measured 

 according to the method indicated for the biaxial system, each 

 layer being investigated separately. 



More properties of the shells are easily measurable, for instance : (i) in 

 the spiral shells, the height of the shell and its diameter at the place of the 

 greatest breadth ; (2) in the conic shells (Patella, etc.), the height, the length 

 and the breadth ; (3 ) in the Bivalves, the length and the breadth of the 

 valves and the transverse diameter (the valves being closed and taken as 

 a whole). 



The properties mentioned above are, of course, compound properties. For 



^ In the majority of the Gasteropods and Cephalopods (Ammonites, Nautilus, 

 etc.) the shell has a spiral form. Here the anticlinal axis NS, crossing the free 

 margin of the aperture of the shell at right angles, is a spirally curved line 

 as the shell itself. The periclinal axis EW is tangent to the margin. In 

 Patella, Ancylus and other similar Gasteropods and also in the Pelecypods, 

 the periclinal axis is concentric (tangent) and the anticlinal axis is radial with 

 regard to the shell considered as a whole. 



H 



