Clarke — The Naples Fauna. 



81 



Manticoceras tardum. 



The breadth of the whorl section indicates relative immaturity, while the 

 long continued duration of the simple varices shows that the nepionic 

 conditions in Mantic. Pattersoni were here greatly protracted. The wide 

 umbilication indicates, to us, an uninterrupted continuation of the primary 

 umbilication manifested directly after the embryonic stage, rather than the 

 increase of umbilication which, in Mantic. Pattersoni, appears with the close 

 of epheby and during gerontic growth. This interpretation is in accord with 

 the elementary expression of the ornament, the whorl section and the 

 septation. 



Manticoceras simulator. 



The expression of the standard species is here followed quite closely 

 save in the immature character of the septation, the obtuseness of the umbilical 

 lobe in all ephebic stages evincing a decided lagging of development. 



Manticoceras rhynchostoma. 



To restate our interpretation of this shell in its relation to Mantic. Pat- 

 terson i: Almost fully like results at maturity have been attained by both 

 through very distinct phases of growth ; while this growth might, in direc- 

 tion, be represented by parallel lines, in rate of development it must be 

 represented by lines of unequal length and with deflected termini. The 

 species is found in a part of the Intumescens province where Mantic. Pat- 

 tersoni is not known to occur. 



Manticoceras contractum. 



The whorl section is nearly that of the standard, if somewhat broader 

 and less progressed. The narrow umbilication we interpret, as in Mantic. 

 appriniatmn, to be an extension of the centripetal tendency which sets in at 

 the sudden termination of the ananepionic maximum of umbilication. The 

 ornament composed of fascicles of low varices suggests acceleration, as in 

 Mantic. fasoiculatum. 



Manticoceras fasciculatum. 



As in Mantic. tan/tan. the breadth of whorl section indicates immature 

 attainment, though in fully mature conditions a palpable narrowing obtains. 

 The wide umbilication we regard as in the preceding species, indicative of a 

 prolonged ananepionic condition, -while the ornamentation shows rapid accel- 

 eration in the brief existence of simple varices, their immediate duplication 

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