of the Transition Formations of the Rhine. 13 



stinction of species, where, as in this case, the height and 

 the breadth increase in entirely different ratios. A necessary- 

 consequence of the more rapid increase in height than in 

 breadth is, that according to the law of geometrical progres- 

 sion, the thickness in the outer w^horls diminishes very ra- 

 pidly also, and it follows that the ammonite in its growth 

 takes a form more and more discoid. In the A. subnautilinus 

 the difference between the increase of height and that of 

 breadth is already great enough to render this character very 

 evident. It appears to me that the fragment described by M. 

 von Buch, under the name of A, evexus, ought to be restored 

 to the present species ; it presents no character that per- 

 mits the establishment of a specific distinction between them. 

 In the A. subnautilinus, as in this, the transverse plates are 

 elevated in the middle, and their greatest depth found to be 

 on the edges near the lobes. 



The A, subnautilinus is met with in the limestone of the 

 Eifel near Gerolstein, and in the state of pyrites in the clay 

 slate of Wissenbach [TJionschiefer). From these two localities 

 I have at present seen only casts. The pyritose fossils of 

 Wissenbach are almost always in the state of casts, and if any 

 striae are visible they must be considered as belonging to the 

 interior side of the shell, which was probably very thin. 



2. Ammonites lateseptatus, n. s. PL I. figs. 1, 2, 3, 4. 



The dorsal lobe infundibuliform ; not much deeper than 

 broad. There is no lateral lobe properly so called, that lobe 

 being only indicated, in the early period of its growth, by an 

 imperfect curve in the septa of the chambers. The increase 

 in height is 070, the increase in breadth 0*65. There are but 

 10 or 1 1 chambers in one whorl. The number of whorls is 

 7, only a third part of the interior whorls is visible. 



This ammonite is found with the preceding, in the clay 

 slate of Wissenbach ; it is well characterized by its form and 

 by its lobes. As the height does not increase faster than the 

 breadth, but rather more slowly, the thickness of the ammo- 

 nite is not diminished in the exterior whorls, but is even some- 

 what increased. Figs. 1 and 2 of PL I. represent the finest 

 example that I possess ; there is very nearly a whorl and a half 



