Clakke — The Naples Fauna. 



71 



MaNTICOCEKAS FASCICULATUM, Sp. nOVJ 

 Plate VI, Figs. 13-22. 



This is another clearly distinct species w hose immature stages are very 

 fully known, while its adult or ultimate condition is still somewhat obscure. 

 The etchings from the Styliola limestone have furnished but a few barite 

 replacements of shells which have assumed mature characters, but have 

 afforded a goodly number of examples in elemental stages of growth. The 

 character of the ornament serves to mate all of these as one species, and 

 while there is still wanting a clear conception of the form of the suture at 

 epheby, this is evidently closely approximated by that last exposed in the 

 largest examples. 



The following features will probably permit the ready identification of 

 the species. 



Shell widely and rather deeply umbilicated, the umbilication at the 

 beginning of the fifth whorl being about three-fifths the diameter of the shell. 

 This is, however, a feature subject to some variation, but the species is at 

 once distinguished in this respect from its allies, J/, a/pprimatum. and J/. 

 Pattersoni, though closely resembling herein M. tardum. 



The later volutions have a broadly sagittate section, the \ enter being 

 rounded and the sides gently convex ; the umbilical slope is very abrupt. 



The suture has a simple manticoceran form, the lateral saddles being 

 comparatively huge, inclined tow ard the umbilicus, and narrow or subacute 

 at their extremities; the ventro-lateral lobes are long and acute; the ventral 

 saddles are well developed and the umbilical lobe is a broad and simple curve. 



The Immature Shell. Protoconch. This is frequently preserved either 

 free or in connection with later parts of the shell. It has about the same 

 >ize and very much the same form as that of Mantic. Pattersoni. Toward 

 its distal end it is inflated, so that the sides of the protoconch protrude 

 noticeably beyond the edge of the first whorl. Viewed from its distal 

 surface it is transversely ellipsoidal; seen from the opposite side it is broadly 

 clavate. 



TJie Conch. At the first overlap contact of the protoconch with the 

 conch the latter has considerably less diameter than the former and in section 

 is shallow and very broad, the venter being depressed, but not flat. The 

 relative increase of the conch in depth (dorso-ventrally) is extremely slow, as 

 will be seen from an inspection of the accompanying figures, and not until 

 the beginning of the fourth volution is there a decided tendency to alter the 

 form of the cross-section to that presented in the more nearly mature shell. 



