Clarke — The Naples Fauna. 



73 



Figures 51, 52. Manticoceras Jasciculatum. 

 Fig. 51. Median section of first two whorls 

 x 12. Fig. 52. Dorsal surface of part of first 

 whorl, showing dorsal lobe, x 25. 



Tht Suture The suture of the first septum makes a nearly transverse 

 course across the conch, the broad outward curve forming only a faint 

 saddle, but a moderately deep and sharp ventral lobe is produced. 

 The edges of the septum do not come together to enclose this lobe, 

 that is. the sipho lay against the wall of 

 the conch at its passage into the proto- 

 conch, and its course over the inner wall 

 of the latter is seen on many specimens 

 as a long scar of the caeca! extremity of 

 that organ. 



.V similar trace of the sipho may often be 

 seen on internal casts of the first two or three 

 aii' chambers. 



The second septum shows a very decided development of the lateral 

 saddles and a sharper development of the ventral lobe; at this stage of 

 growth the course of the septum on the dorsal side of the whorl is almost 

 precisely that on the ventral, except that the dorsal lobe is slightly less 

 developed. In the course of the second ami third whorls there is a constant 

 increase in the size and prominence of the ventral lobe, which then occupies 

 fully one-half of the broad ventral surface of the whorl, the lateral saddle 

 becomes more prominent and occupies the shoulder of the volution and the 

 lateral lobe of about the same size occupies the lateral exposure of the whorl, 

 recurving at the nmbilicusto form alow saddle. There is no modification of the 

 large ventral lobe until the fourth whorl, where its late- 

 ral slopes become divided and the ventrolateral lobes 

 are formed, with a narrow, intervening ventral saddle. 



On the dorsal side of the whorl the development 

 of the suture continues nearly jxiri jxissii with that 

 on the exposed surface, up to the opening of the third 

 whorl. Thereafter follows a rapid increase in the 

 prominence of the lateral saddles which is not equaled 

 by that of the umbilical saddles. In the course of 

 the third volution the dorsal saddle is divided forming 

 two small saddles and an intervening lobe, and the 



median (lol'sal lobe becomes Very milch naiTOWed. Tile Figure63. Manticoceras fascicula- 

 1 . . . . liii t • • turn. Ventral view of the first half 



suture up to this point has attained all the divisions of third whori, showing the corm- 



.... • i> ii i i i gations and the course of the septa. 



which it presents in fuller development at later stages. X 25. 



While we have referred this species to Manticoceras it is not done 

 without reservation. The mature form of the shell is in harmonv with 



