OF PLANORBIS AT STEINHEIM. 71 



Another interesting peculiarity is the obUteration of the upper umbilicus in several 

 turbinate forms, as in fig. 1, line d, and its approximate obliteration in figs. 2 and 6, line 

 c. It must be understood, however, that even in fig. 1, line d, there is a minute upper 

 umbilicus in the extremely young shell, as seen at the apex. 



A very cm'ious tendencj^ in the whorl to depart from the regular mode of growth is 

 apparent in figs. 6-9, pi. 3, line p. The whorl, either through a wound as in fig. 8, received 

 at an earlier age, or through some weakness caused by sickness or unfavorable conditions, 

 ceases to increase by growth according to the usual proportions. This contraction grad- 

 ually leads to the distortion of the spiral. Sandberger figures one of this species much 

 much more remarkable than any here. The mouth strikes off almost as a tangent to the 

 curvature of the spiral, and extends out to a distance very much beyond that of any of 

 the specimens seen by me, except perhaps fig. 5, line h, pi. 8. 



The distorted forms to whose illustration plate 8 is devoted, are largely taken from 

 PI. oxystomus, and it will be observed that most of them are var. revertens Hilg. = "^uSu"^^- 



Figs. 3-4, and perhaps 5, line a, figs. 1, 6, line h, are from specimens of typical PI. 

 oxystomus, while figs. 2-5 line h, and all of line c, are taken from '"'^i'^^^ . Some of 

 these cases are evidently due to wounds, the eflfect being distinctly marked on the shell. 

 That the wound in such cases affected the health of the animal is evident, because in other 

 cases of shells similarly scarred, no distortions are observable in the subsequent building 

 up of the shell. Figs. 3, line a, 2, line h, and all on line c, are undoubtedly due to such 

 accidental causes. The other shells show no derangement in their striations or scars. 

 These distortions may or may not have been due to diseases arising from other causes, but 

 of one thing we can rest assured, that """^ifj,"™"' was more subject to such distortions than 

 any other species or form belonging to any of the progressive series. If it is desirable 

 to test the conclusions drawn from such diseased specimens in chapter 1, it can be 

 readily done by comparing figs. 1-3, line c, and 4, 5, line b, with the uncoiled forms 

 of the retrogressive sub-series, or better still by observing the close parallelism of fig. 5, 

 line b, with fig. 23, of pi. 9. 



Another remarkable result of disease, whether it may be from accident or otherwise, is 

 a reversion to the peculiar angularity of form conspicuous in some varieties of PL levis. 

 This is not very well shown in the plate on account of the positions of the specimens, but is 

 partially shown in fig. 2, fine b, which was especially intended to exhibit this peculiarity. 

 It results from a perceptible flattening of the diseased portion of the whorl, as well as from a 

 general diminution in size. The diseased specimens of PL supremus, figured on line d, 

 are very large, and they are distorted only towards the latter part of the last whorl. The 

 distortions consist of enlarged striae, and simultaneously the size of the whorl decreases, 

 occasioning at once a deflection from the regular increment of the spiral, which tends to 

 become turbinate.^ 



These phenomena appear to indicate the slow rate of growth consequent upon old age. 

 This is also shown very well in fig. 1, line n, pi. 3, when the same enlarged striations 

 are seen on the last part of the last whorl, and it is this part alone which is deflected to 



1 1 do not wish to be understood as implying that the ten- equally unquestionable that PL irochiformis is a thoroughly 

 dency to become turbinate is always a, diseased or retro- healthy species, as are many other turbinate forms, 

 gressive characteristic. While this is often the case, it is 



