OF PLANORBIS AT STEINHEIM. 63 



the shells of PI. minutus, and a very large proportion of PI. Kraussii appear to have 

 been subjected to the action of an acid sufficiently powerful in many instances to cancel- 

 late the outer surface, and destroy the striae. It is, however, easy to distinguish the per- 

 fect shells after a close examination, though sometimes the striae are so fine and equal, 

 that at first sight, even under a magnifier, the shell appears to be absolutely smooth. 



The costae are not found on some specimens of PI. Kraussii or PI. minutus, but 



in others they are distinct, though in the latter more difficult to see than in the 



former, on account of the size of the shell. The costas in their turn are apt to 



be confounded with the still larger and more opaque ridges left by the building in of 



mouth rims, which have marked long periods of arrest of growth in the shell. I have 



not been able to reduce them to any law on account of the perfect way in which 



they are generally covered up when growth is resumed. In recent species of Planorbidae 



it is quite possible to trace them occuring at regular intervals by their opacity and color, 



and they are evidently due to seasonal arrests of growth, but in the fossils they 



are too readily confounded with the striae, though occasionally noticeable. When 



the shell attains its growth, however, the arrests of growth appear not to be wholly 



limited by the seasons. The buUding period appears to be shorter and more irregular, 



and in distorted specimens this is particularly noticeable. See, for illustrations of this, the 



figures of PI. supremus, pi. 4, already described, and the following, fig. 9, line g, fig. 13, 



line h, pi. 1 ; figs. 2, 4, line c, fig. 5, line m, pi. 2 ; figs. 1, 7. line g, fig. 1, line n, pi. 3 ; 



figs. 3-5, line a, figs. 1-4, line d, pi. 8, all simple forms. A noticable case of distortion 



combined with senility is that of fig. 10, line r, pi. 2, which is repeated on hue e, fig. 2, 



in a different position. 



These were not specially selected to show these peculiarities, but are very good 

 ordinary examples of the pathological conditions described. In any shell there may 

 be every condition from that of the young or full-grown healthy shell marked with 

 striae, costae, and permanent mouth-rims, to its old age form, in which the costae are 

 susperseded by permanent mouth-rims occurring at rapid intervals, and finally to 

 the last stage of debility, in which the latest buUt mouth-rim projects only slightly 

 beyond the former and greatly narrows the aperture, as in fig. 4, line c, pi. 8. The 

 thickness of the permanent mouth-ring varies greatly in different adult individuals, 

 even of species like PI. oxystomus, which habitually have a very thick lip in the 

 adult. As a general rule, however, the moiith-rims are thinner in the young shells 

 of all forms, whether species, or varieties, or individuals, than in the adults ; and 

 especially so in those which thicken the lips or rims during their subsequent growth. 

 This peculiarity aids in the concealment of the cicatrix or ridge of the permanent 

 mouths during growth, so that these become apparent in most^ shells from the 

 Steinheim Pits, only when the edges have been broken during the season of rest, or 

 after the sheU has reached the full adult size, and forms a thicker rim than is usual 

 in the young. 



The first of the series of the costate forms are not distinguishable from the 

 typical PI minutus, or from the varieties intermediate between that species and 

 PI. denudatus, except by the presence of distinct fine costae. These do not occur 

 in the young shells of the forms most closely allied to PI. minutus, but only on 

 the last whorl in the full grown adult shell. This point I have established by 



