of the Transition Formations of the Rhine. 15 



the suture, but not quite to the height of the dorsal saddle. 

 The increase in height is 0*28 ; the increase in breadth 0*5. 

 There are 1 8 chambers in one whorl. The interior whorls 

 are not at all enveloped, but entirely free. The only specimen 

 that I am acquainted with is in the beautiful collection of M. 

 Dannenberg at Dillenburg; the figure is taken from a model 

 in plaster. It is a fragment of which only two whorls are 

 preserved; it wants the interior whorls and the outer un- 

 chambered part. The form is completely discoid, from its 

 more rapid increase in height than in breadth. This am- 

 monite is thus distinguished from the A, expansus, which has 

 the whorls entirely enveloped. In the latter the height in- 

 creases still more rapidly, yet has less disproportion with the 

 increase in breadth. If we suppose that in this ammonite, 

 according to the ordinary law, there is a whorl and a half with- 

 out chambers, it will be found from the proportion of the 

 increase in height, that the diameter of the entire shell would 

 be nearly a foot. The thickness ought in this species to 

 diminish very rapidly, since the breadth of the mouth in- 

 creases much more gradually than the height. The dimension 

 is, at the commencement of the first of the two whorls pre- 

 served, 0*7 ; at the commencement of the second, 1 ; and at 

 the termination of the second, 1'5. The greatest thick- 

 ness is at the middle of the side ; it decreases however, but 

 gently, till approaching the suture and the back. Upon the 

 outer of the two whorls the back is completely rounded, 

 and passes gradually to the side. Upon the inner whorl 

 it becomes flat, and at the commencement of the second whorl 

 it forms almost a right angle with the sides. Probably there 

 was upon the shell, between the back and the two sides, two 

 sharp edges, the impression of which may be seen upon the 

 cast. These edges limit the inflected curve behind, which is 

 formed by the striae of the shell and the back : they gradually 

 disappear upon the outer whorls. We shall see in many of 

 the following species this diflerence in the manner in which 

 the back is united to the sides, in the exterior and interior 

 whorls. The lobes of A. Dannenbergi are not essentially 

 different from those of A. subnautilinus. The dorsal saddle 

 is always somewhat narrower and higher ; the lateral lobe, on 



