62 HYATT ON THE TERTIARY SPECIES 



This appears also to have been the result arrived at by Sandberger, though from 

 his conclusion, I must also differ in part, on account probably of the opportunities 

 afforded by a larger amount of material, which has enabled me to trace the connection 

 of costatus with "'j^^f '. 



Flanorbis costatus. 



Plcmorbis costatus Klein, OjJ- cit., pi. 1, fig. 24. 

 PL midtif. costatus Hilg., Op. cit., fig. 18, 18 a. 

 PL costatus Sand., Op. cit., p. 647, pi. 28, fig. 5. 



'XTny costatus 

 ' "* ' minutus' 



In order to understand what follows it becomes necessary to trace the relations of the 

 striae of srrowth, and the costse of the shells. It will be observed that the striae of 

 growth are of various degrees of fineness and prominence in all the species of the Stein- 

 heim Planorbidae. Sometimes they are hardly observable, since they are not prominent 

 enough to be seen with the unassisted eye, though visible with a magnifier of four diam- 

 eters, as in PL minutus and denudatus. There are all degrees of this in PL Stehiheimen- 

 sis for example, until we find specimens in which they are plainly visible by the naked 

 eye. In other species, such as PL tenuis, discoideus, trochiformis, oxystoynus, and supjre- 

 mus, they are distinct enough in many specimens to catch the light, and be visible in the 

 photographs, and in all specimens with the naked eye. 



This is also an effect of old age, as in the older portions of the whorl of the senile 

 specimens of PL supremus, fig. 1-4, line d, pi. 8. In these, and in many others, any 

 cause which retards or arrests growth, causes an increase of shell deposits at the lines of 

 growth, and a consequently greater prominence of the striae. That these more promi- 

 nent striae may be also a hereditary characteristic in perfectly healthy shells is demon- 

 strated by such series as PL lecis to PL crescens, and others, and by PL discoideus var. 

 sulcatus, etc. 



In PL crescens they are visible under a magnifier, as in PL Steinheimensis and PL 

 levis, and in none of these, or in equally finely preserved shells of PL minutus, is there 

 any tracable difference except in point of fineness. This fineness also differs in different 

 shells of the same species, precisely as it does between different species, being finer in 

 some than in others, according to the rate of growth of the animal. Between each pro- 

 jecting ridge or striation there is a (usually, but not invariably) sunken smooth 

 band of exceedingly variable width in the same shell. At intervals there are striae, more 

 elevated than the rest, more elevated than those on either side of them, which are the 

 costie proper. They are formed by an arrest of the growth at this point, occasioning a 

 slightly greater thickening of the shell. The rim of the opening not being absorbed in 

 these shells, when growth is resumed at the usual rate of increase, and the true striae 

 begin to be again formed, there remains a larger and more prominent ridge. These costiB 

 will often occur is some specimens of a species, and not in others. They are quite rare 

 in those species which, like PL Kraussii, have very fine striae of growth, but can be dis- 

 tinguished in some specimens. Care must be taken in both this and PL minutus to 

 obtain shells which have not been acted upon by any re-agent. The larger number of 



