Clarke — The Naples Fauna. 



93 



to and probably through the fourth volution and thence onward a rapid loss 

 of overlap and increase of umbilication. In Mantic. Pattersoni we observed 

 that this change from gain to loss of overlap occurred apparently in a late 

 ephebic substage, but in this species this change is induced much earlier, 

 before we should regard the ephebic condition as fully introduced. 



OrnarneiUation. The barite replacements which have preserved the most 

 delicate ornamentation in other forms, are here all smooth up to the close of 

 the fourth volution or show only faint concentric lines. Calcite removals * 

 show to better advantage such ornament as the species possesses. 



Figures on Plate VII show that for nearly one and a half volutions the 

 whorls are smooth, directly thereafter comes in a series of simple, low, elevated 



73 74 



Figures 73, 74. Probeloceras Lutheri. Front and lateral views of a 

 young shell of two volutions. Fig. 73 shows the prominence of the 

 double ventral keel ; fig. 74 shows the first and three of the later 

 septa, x 25. 



striae, the first of which seem not to extend to the umbilical margin, but those 

 succeeding extend entirely across the exposed parts of the whorl. These 

 become too faint t<> be seen shortly after the commencement of the fourth 

 volution, show ing at no place an intercalary series. "Where the entire surface 

 of the whorl is exposed it is seen that these lines extend with the faint curve 

 indicated, to the venter, where they become abruptly elevated as they pass 

 over the ventral flattening. In the specimen (barite) represented in figure 2 

 showing just two volutions, the strong lamella? made by these lines at the 

 ventral surface are shown, although the lines in their extent over the lateral 

 slopes of the w horl are not retained. In viewing such a specimen from in 



* A calcite shell embedded in an impure, argillaceous limestone such as composed the concretionary masses abounding 

 in the Portage beds, upon being completely dissolved out by acids, leaves an exact intaglio which will show the finest 

 detail of surface markings in case the specimen was in good condition before fossilization. The matrix, after its acid 

 bath, is soft and clayey but upon being thoroughly washed and dried may be hardened by soaking in weak glue. 

 After a second drying, the specimen will permit an impression in gutta percha, if made quickly. Such impressions are 

 referred to as " calcite removals." Many figures on our pages and plates are from these. The writer has found this 

 simple process marvellously successful in the elaboration of structural details in many fossils. 



