Clarke — The Naples Fauna. 



53 



to wholly embrace that preceding ; it is measured by the degree to which each 

 whorl is overlapped by that following. We have seen that in the earlier 

 volutions of Mantic. Pattersoni, the overlap for the first two or two and one- 

 half volutions is very slight. From the outset the dorsal impressed zone 

 is clearly developed but its increase in depth is very gradual ; at first, 

 where the primary whorl is attached to the protoconch, being little more than 

 a surface of apposition. The actual diminution of umbilication with growth 

 can be expressed in terms of the whorl overlapped. Thus in a well developed 

 adult of 6 1 volutions it is as follows: at the end of the second volution one- 

 fourth of the dorso-ventral diameter of the whorl is thus overlapped ; at the 

 end of the third, three eighths ; at the end of the fourth, seven-tenths ; half way 

 through the fifth, nineteen-twenty-fourths ; at the end of the fifth, seventeen- 

 twenty-sevenths. In other terms expressed approximately; 6.5+, 10—, 18.5, 

 20.5, 16 + . Thus it appears that up to near the completion of the fifth 

 volution there is a gradual increase of overlap or a consequent diminution 

 of umbilication and thence onward the degree of overlap gradually declines 

 and umbilication actually increased. Decrease in umbilication in the 

 forms we have in hand certainly indicates approximation to a phylephebic 

 condition. The very evidence which this species affords demonstrates 

 that gradual loss of umbilication is a process persisting from the ananepionic 

 condition to maturity. It is therefore the gradual assumption of a mature 

 character. The comparatively abrupt reversal of the process, involving 

 an increase in umbilication throughout the course of the very last whorl, 

 may be regarded as coincident with gerontic growth and is evidence of decline ; 

 decline not alone of the individual nor of the specific type, but a phyletic 

 result pronounced in well known instances from the early nautiloids to the 

 vanishing expressions of the ammonoids. This evidence of such unwinding 

 in the goniatites, though not striking, is corroborated in the other species here 

 discussed, while in the highly involute forms of Toniocera-% especially illustrated 

 by T. unia/ajualare, the umbilication is wholly lost very early and not mani- 

 fested in any later growth stage. 



Ornamentation. We have observed that the assumption of concentric 

 lines of ornament opens the neanic stage of shell growth. Hitherto the shell 

 has been smooth. 



The first ornament-lines are relatively strong and distant suberect lamellae, 

 appearing only over the ventral surf ace, not extending from one lateral furrow- 

 to the other ; that is, not encircling the exposed portion of the whorl. 

 Two or three such short lamelhe may introduce the longer ones, and when 



