1887.] 



ON CYCLESTHERIA HISLOPI. 



11 



umbones forms a bold curve in the middle, in some specimens even 

 approximating an angular bend, and its posterior part descends as 

 a rule almost perpendicularly, joining the free edges of the valves 

 without forming any distinct angle. In only a few specimens. ex- 

 amined in the beginning of September (PL II, fig. 2). was a dis- 

 tinct angular bend fonnd marking off the posterior free edges of 

 the valves from the dorsal margin, which latter, moreover, exhib- 

 ited a more regularly vaulted form. The free edges of the 

 valves are thronghont evenly curved. and quite smooth, without 

 any trace of marginal spines or bristles. The lines af growth 

 observed on the surface of the valves. vary in number accord- 

 ing to the age of the animal, and are very delicate (not so 

 coarse by far as indicated in the figures). They are arranged. 

 as usual, in a concentric manner, the first encircling the um- 

 bones, the others occurring at some distance from one other. The 

 intervals between the lines increase as a rule successively in size; 

 but in some specimens (Pl. II, fig. 2) the two or three last lines 

 of growth, running parallel to the edges of the shell, are rela- 

 tively much more approximate than the preceding. In the largest 

 specimens. I have counted as many as seven lines of growth. and 

 the same number has also been recorded by Baird in his Indian 

 specimens. though the figure of the shell given by that author. 

 apparently from some error of the lithographer, shows the number 

 to be much greater. As seen from above (Pl. I, fig. 2). the shell 

 appears somewhar tumid, though the greatest breadth which occurs 

 about in the middle, does not quite attain one-half its length. 

 In young specimens (Pl. VIII, fig. 4), the shell exhibits a more 

 oval form, the height being not by far so great as in full-grown 

 specimens, the umbones too less prominent and the dorsal line but 

 very slightly arched. The shell of full-grown specimens attains a 

 length of 5mm which is also the size given by Baird in his plate, 

 whereas the specimens examined by Brady were considerably 

 s maller , and in all probability still immature. It is of a clear 

 yellowish or corneous colour, with the surface rather smooth and 

 lustrous. On the whole, it very much resembles the shell of a bi- 

 valve Mollusc, a Cyclas. for example, or still more maybe a Pisidium. 



