GENERAL MORPHOLOGY. 5 



(i) The sides are straight or slightly concave throughout the length of 



the shell, e.g. C. laevigata. 

 (ii) The straight lines are replaced towards the apex by convex curves, 



while the shorter axis becomes still more diminished, giving rise 



to a flattened ellipse or figure CO, e.g. G. soioerbyi, C. vesicularis 



(PL V, fig. 4 6). 

 (iii) The straight lines are replaced by convex curves throughout, giving 



rise to an ellipse, e.g. G. complanata (PI. IV, fig. 13). 

 In one somewhat doubtful case the sides appear to be equal in pairs, giving 

 rise to a rectangle, e. g. G. punctata (PI. I, fig. 10 b). 



(3) Faces of the Pyramid. — Except in the doubtful cases of G. laevigata and 

 G. punctata among British species, the faces of the pyramids are equal, and 

 similar elongated triangles, with surfaces either plane or with the modifications 

 mentioned above. The apical angle of the face can frequently be measured, even 

 when the specimen is very imperfect. It is usually found to have a fairly constant 

 value for a species, and is therefore of considerable specific importance. It is not 

 readily affected by compression of the shell, as is the case with the apical angle of 

 the pyramid, hence the values given are generally reliable. When the shell tends 

 to become prismatic towards the aperture, the angle of the face becomes corre- 

 spondingly less. 



(4) Longitudinal Grooves <ni<l Ridges. — The faces are separated from one 

 another in all cases by straight, well-marked, longitudinal grooves, which run 

 down the angles of the pyramid. These " marginal grooves " vary greatly in 

 character, but are very constant for a species. In all the earliest forms they are 

 smooth, deep, and with a tendency for the edges to become prominent, as in 

 G. coronata (PI. Ill, fig. 1 a) and G. homfrayi. In the later ornamented types 

 the ornamentation of the faces may be continued across the grooves without change 

 in direction, as in 0. soioerbyi (PI. V, fig. 10 a) ; or it may change its direction at 

 the edges, and cease at the base, as in G. quadrisulcata (PI. Ill, fig. 2) ; or it may 

 cease abruptly at the edge, leaving a smooth groove, as in G. breviconventa (PI. V, 

 fig. 13). 



In shape also there are all stages, from a sharp, well-defined groove, such as 

 is seen in G. hispida (PI. Ill, fig. 9 a) ; to the wide, shallow undulation seen in 

 G. crassa (PI. IV, fig. 4 "). 



On the surface of the face itself longitudinal grooves are sometimes seen. In 

 the earlier smooth forms these " facial grooves " are constant, well marked, and 

 frequently resemble the marginal grooves in having prominent, raised edges (e. g. 

 G. coronata, PI. Ill, fig. 1 a) ; but in most of the later species, if present at all, 

 they appear as narrow depressions, or frequently as fine cracks, marking the line of 

 weakness, along which the transverse ridges are bent. They are always central or 



