184 Prof. T. Rupert Jones on 



shapes. In some cases the dorsal notch is nearly obliterated 

 by vertical pressure ; and in other instances it is exaggerated 

 by horizontal squeezing, when the valve becomes shortened to 

 a small subglobular mass, and the two ends of the valve are 

 made to approach each other. 



The real outside of a valve is very rare. The convex cos- 

 tulate specimen (fig. 8), even, may be destitute of the external 

 coating. Hollow impressions of the surface, sometimes with 

 remains (or mineral replacements) of the test adhering thereto, 

 are common ; and the interior of these moulds usually has the 

 ridge and knob, or some indication of them, which are due to 

 the nuchal incurving of the test. It is possible that on the 

 exterior this feature was not nearly so strong as within, being 

 due as much to a local internal thickening of the substance 

 as to a bending-in of the shell. It was, however, always 

 sufficiently defined to form a weak line when the valves 

 were shortened by being squeezed endwise (see figs. 3, 

 15, 16). 



. The convex casts of interiors are destitute of riblets, and 

 sometimes (in the Saalfeld specimens) bear only a central pit, 

 corresponding with the central knoblet of the hollow rib- 

 leted moulds. Thus these insides of valves appear to have 

 been smooth, with a small central tubercle, and no definite 

 trace of the external nuchal sulcus, which, however, gives a 

 distinct mark on internal casts of valves bent and shortened 

 by terminal pressure, as mentioned above. In specimens 

 from other localities the smooth convex casts bear the nuchal 

 furrow and no definite central pit. 



None of the smooth convex casts with a pit only are 

 figured here, but are recognizable in some of the published 

 illustrations, as in Richter's ' Beitrag ' &c. pi. ii. fig. 2. 



Many similar smooth convex casts, but bearing the nuchal 

 furrow, are present in a drab mudstone of the " Cypridinen- 

 Schiefer" from Nassau (formerly in Mr. Daniel Sharpe's col- 

 lection), and are oblong-oval, narrow, oblique, or quite 

 round*, according to the direction of the lateral (horizontal) 

 pressure to which they have been subjected. 



The longitudinal wrinkling of the surface of the valves 

 varies in the number, arrangement, smoothness, and thickness 

 of the riblets or costula 3 ,. They may be quite even and smooth 

 (fig. 18 b), slightly roughened (figs. 6 b, 7 b, and 8 b), crenulated 

 (figs. 1 b and 5 b), and even serrated (fig. 4 b ; see also ' Zeit- 

 schr.' 1869, pi. xx. fig. 10). The indentations are sufficiently 

 coarse and defined in some specimens of these hollow moulds 



* Such as are figured by Richter, ' Beitrag ' &c. pi. ii. figs. 28, 31, and 

 ' Zeitschr.' 1869, pi. xx. fig. 7. See also our figs. 3, 15, 16. 



