1 88 2.] Transformations of Planorbis. 445 



This explanation is obviously applicable to the regular spirals, 

 but the test cases are the irregular spirals. 



These occur through weakness occasioned by wounds, disease, 

 Fig. 21, PI. vii, or old age, as in Fig. 22, PI. vn. 



All of the distortions thus produced tend to be irregular, that 

 is the animal becomes too weak to counteract the effects of the 

 weight of the shell by its inherited muscular power, and it falls 

 over more or less to one side destroying the regular curve of the 

 spiral. This falling over to the side of greatest weight occasions 

 an irregularity in the deposition of the outer shell layers, and the 

 shell becomes more and more irregular as the animal grows 

 weaker in old age or through disease. 



Another proof of the effects of gravity lies in the fact, that the 

 irregularity of form of the shell is proportional to the extent to 

 which it is supported and the excreting border of the mantle 

 relieved from the effects of its weight. 



Thus, a perfectly regular spire in the young, by being supported, 

 is turned into an irregular meandering tube in the after growth of 

 the same animal. 



Magilus antiquus crawls freely when young among corals, and 

 has, during this period, a regular turreted shell, Fig. 23 da, PL vn. 

 It becomes finally fixed in the growing coral, which completely 

 invests and supports it, and thereafter its shell is a rough irregu- 

 lar tube growing upwards in the direction of least resistance. The 

 border of the mantle being free from compression on all sides, 

 deposits shell matter about equally all around in the specimen 

 figured, and therefore grows upward in a straight line. 



The Vermetidae are supported in various degrees in the adults 

 but free in the young. Their shells, therefore, though having a 

 regular spiral in the young, are in proportion to the support re- 

 ceived, transformed into tubes more or less meandering after they 

 become attached as in Fig. 10 a, PI. vn, or loose irregular spirals 

 rising up like corkscrews and only supported on one side as in 

 F 'g- 10. 



The Ammonoids and Nautiloids are notable for the complete 

 bilateral symmetry of their spiral shells. 



Diseased specimens, which are not infrequent, however, tend 

 to become unsymmetrical, coiling like the Gasteropods, as is well 

 known to all experienced palaeontologists. These are commonly 

 spoken of by Quenstedt and others as diseased or deformed or 

 sick'species. 



