INSTITUTE OF SCIENCE, PHILADELPHIA. 363 



usual, are apt to be smaller and, not having to carry the enormous egg-sack of 

 the females, have the " shoulder " of the shell, or that part of the whorl just in 

 front of the suture, less inflated, giving the whole shell a more evenly conical 

 and less scalar spire. These differences are more marked in the group hav- 

 ing a corneous operculum, but are perceptible in the others, especially those 

 with an elevated spire. 



Apart from sexual differences, there is a certain variability about the coil 

 of the shell, some specimens having a decidedly wider umbilicus than others 

 of the same species, and the grooves and spiral ribs of the interior of the um- 

 bilicus vary within certain limits between individuals and also have a certain 

 range of fluctuation in the same individual at different times, since the growth 

 of the margin of the outer lip, and of the inner lip, including the end of the 

 umbilical rib or ribs, do not always preserve an undeviating ratio to one 

 another, a deposit apparently taking place on the one or the other at dif- 

 ferent times and not always coincidently. 



In treating of the fossils, too, it should be remarked that the outer layers 

 of the shell seem to be less intinnately soldered to the inner ones than in many 

 other groups, and therefore it is not uncommon for a fossil specimen by de- 

 cortication to lose its normal aspect entirely, without appearing to lose its 

 symmetry, the result being often very deceptive. This is especially the case 

 with Ltinatia, Neverita, etc., and is not so marked in the forms belonging to 

 Natica proper. As between youth and old age, a very marked difference may 

 sometimes be observed in the umbilical characters of specimens of the same 

 species. 



In the group oi Ampullina young specimens sometimes show strong, 

 rounded ribs, or a narrow but distinct sulcus, bounding the umbilical fasciole. 

 In the adult these characters may become so obsolete that only by means of 

 a series of different ages is one able to determine that they belong to the same 

 group. One such form when quite young seems almost like the type of a 

 new section, and when full-grown can hardly be distinguished frorrj an ordinary 

 Lunatia by the shell characters. The same species may be found to have the 

 umbilicus bounded by a sharp elevated thread, or by an equally sharp groove, 

 and the gradual modification of a groove into a thread — if such an expression 

 may be used — is one of the most interesting and instructive object-lessons on 

 the mutability of specific limits. It maybe admitted that most of these muta- 

 tions are of Eocene age, and that, in the lapse of time since that formative 

 period, the specific characters of the forms which have survived have become 

 more definitely marked. But even now the forms with a corneous operculum 

 exhibit a wide range of variation. That those species with a shelly operculum 

 are less variable, I regard as the direct result of the dynamic influence of the 

 rigid operculum itself on the soft parts of the animal which bears it — a view 

 upon which I cannot expatiate here, but which opens up an interesting and 

 almost virgin field for inquiry. 



