Clarke — The Naples Fauna. 



51 



less than that of the protoeonch. It is also broader in its surface of attach- 

 ment to the protocorich. The entire growth of the nepionic 

 shell is not a regular diminution in size, but the least 

 dimensions are attained with the close of the preponderating 

 convexity of the early stage. This is shown in many of 

 our figures where it is seen that the median portion of the 

 nepionic shell is more fully enveloped by the succeeding 

 whorl than any other part of it. The latter part of the 

 nepionic stage is again a condition of increase in size but Figured. Theprotoconch, 



, nepionic and neanic whorls, 



an increase less rapid than the decrease of the early stage, in an adult specimen of 



Mant icoceras intuviescens. 



Thus are denned with unusual lucidity the ana-, meta— This specimen is remarkable 



for the great expansion of 



and paranepionic periods of growth. The ne i)ionic the anane P ioniC8he11 which 



x A ° -*• is much more pronounced 



period, usually terminated by the introduction of orna- than in *■ PatUrsoni - 125 

 mental ribs, is sometimes abruptly ended by an annular swelling as seen 

 in fig. 11. 



Let us now observe that the point at which the form of the conch attains 

 most nearly a circular outline, is in the metanepionic substage ; it is, however, 

 only here less broad, not necessarily of more primitive shape than in the ana- 

 and paranepionic conditions. From the earliest period the impressed zone is 

 fully developed and there is no evidence of complete umbilical perforation or 

 entire dissociation of the protoconch from the dorsal wall of the conch. 



With the close of the nepionic stage the form of the whorl is transversely 

 subelliptical, its major and minor axes being as 2 to 1.* 



The form in the Neanic and Ephebio stages. Deferring for the moment 

 any attempt to distinguish the later growth periods of the shell it will be 

 observed from accompanying figures that the proportions of the whorl 

 -cci ions change but lit- tie throughout the first 



three volutions of the 

 relatively great breadth, 

 compression manifests 

 ly at the fourth volu- 



shell, which retain a 

 The tendency to lateral 

 itself somewhat abrupt- 

 tion, the section becom- 



ing obcol'date and Coil- Figures 13, 14. The early neanic shell, x 12. tiliuillg its form through 



the fourth and fifth volutions, its length in the fifth volution being one-fifth or 

 one-sixth more than the width. Thereupon sets in the flattening of the lateral 

 slopes and the broadening of the venter accompanied by a great increase in 

 dorso-ventral expansion. A final condition of this tendency is the projecting 



