8 THE BRITISH CONULARLE. 



position of the shell in life was probably upright, it is difficult, without the soft 

 parts, to distinguish dorsal and ventral sides. In describing the compressed forms 

 I have used the terms "central" and "lateral" marginal grooves for the sake of 

 convenience for expressing, respectively, those at the extremities of the short and 

 long diameters of the cross-section. 



(7) Ornamentation. — A small number of the earlier members of the genus have 

 smooth shells, but the majority show a very delicate and beautiful ornamentation, 

 differing greatly in different species, although usually constant for each species. 

 Among British forms this ornamentation is essentially a transverse one, though 

 in some foreign species a longitudinal direction prevails (e.g. C. scalaris, Holm). 



Each face is tj^pically crossed by a series of ridges, separated by furrows, and 

 bent up regularly along a central or subcentral line, so as to form a succession of 

 chevrons opening towards the apex. The angle of the chevron varies from 180° 

 to little over 90°. Very usually the angle increases in size from the apex towards the 

 aperture, but in any one species is fairly constant for the central part of the shell. 

 The straight sides of the chevron may be replaced by simple or compound curves. The 

 two sides of the chevron are symmetrical in those species having a square cross- 

 section (PI. IH, fig. ( .)a), but where a compressed form is the natural one the 

 ridges very frequently fall away more sharply to the lateral than to the central 

 marginal grooves. With this character is often found a shifting of the points of 

 the chevrons towards the central marginal groove (PI. V, figs. 1 a, 7 a). These 

 features often afford a clue to the natural cross-section, when the latter cannot be 

 directly observed. The ridges vary in width from coarse bars, nearly 1 mm. wide, 

 as seen in (J. crassa (PL IV, fig. -I"), to the finest striee, invisible except under 

 the microscope, as in 0. microscopica (PI. II, fig. 9). In an individual example 

 the ridges usually become finer and more crowded at the apex, and often again at 

 the aperture, but for the centre of the shell a fairly constant spacing is maintained 

 in any one species. 



The details of ornamentation vary enormously. The ridges are sometimes 

 quite smooth, but more frequently the summits are studded with fine tubercles, 

 round, or less commonly elongated, triangular, etc. The tubercles may be pro- 

 longed as fine points on to the intervening furrow, as in C. hispida (PI. Ill, figs. 9 b, 

 11); or may be confined to the ridges, as in G. globosa (PL III, fig. 7 a). In 

 a considerable number of species a well-marked longitudinal striation is seen in the 

 furrows between the ridges ; this is usually finer than the main transverse ridging 

 (PL V, figs. 2 a, 12 6). 



In a few cases the transverse ridges are represented by rows of tubercles, 

 arranged regularly across the shell, as in C. aspersa and < '. punctata (PL I, figs. 7, 

 86, 9a, 12 a). 



(8) Structure of the Shell. — The shell in the British species is always very thin, 

 rarely exceeding 1 mm. in thickness, and often very much less. It appears to 



