Clakke — Tiik Naples Fauna. 



45 



and thus the genus becomes a synonym of the name Manticoceras. Among 

 the other typical species cited by Hyatt is the Got). Buchi, de Verneiiil,* 

 which is throughout of the Intumescens type, most closely approximating the 

 normal of our fauna. The name Gephyroceras has, however, been employed 

 by v. Zittel for species of discoidal form, with wide umbilication and peripheral 

 sulcus such as Gon. calculiformis, Beyr., which Hyatt included in his list of 

 examples, and upon which, it would seem, that the generic definition was 

 largely based. It would probably not be far from the intention of the author 

 of the term to propagate it with this meaning. 



Manticoceras Pattersoni, Hall (sp.). 



Plate I, Figs. 1-12 ; Plate II, Figs. 1-4, 6 ; Plate IV, Figs. 14-18. 



Form of Shell. The Normal Adult. In the Naples fauna the predomi- 

 nant form is of moderately large size at maturity and its adult characters are 

 practically assumed with the attainment of an approximately invariable 

 dimension which, under normal conditions, is a width of a*bout 80-100 mm. 

 The shell is sublenticular, with the final whorl laterally compressed ; its sur- 

 face highest close upon the umbilicus where the slope on the final whorl is 

 abrupt and almost vertical to the next inner whorl, and whence it rounds 

 broadly outward with a gently convex curve to the venter, itself narrow but 

 well rounded. The actual contour of the final whorl, of which the chamber 

 of habitation occupies one-half, is progressively changing throughout its length, 

 as we shall have occasion to observe more fully, and this modification consists 

 in the alteration of the form of the conch by the broadening of the lateral 

 slopes, the lessening of their convexity, the narrowing of the venter and the 

 increasing prominence of the lateral angle. The usual form of the whorl in 

 cioss-section at the base of the living chamber of the adult has a somewhat 

 sagittate outline. 



In this normal form the umbilication is complete, though it is not great 

 either at maturity or in any of the approaching stages. It appears to be an 

 increasingly conspicuous feature as growth advances through the stages of 

 maturity, though the degree of umbilication of the shell is actually decreasing 

 up to maturity. Whatever features, however, are here specified as characterizing 

 epheby must be looked upon as climacteric in their processional acquisition or 

 modification. Variations from the prevailing type have, as we shall observe, 



♦Trans. Geol. Soc. vol. vi, pi. xxvi, flg 1 (not fig. 2). 1842. 



