18 M. E. Beyrich on the Goniatites 



saddle, occupies half the side, and is gently hollowed in ap- 

 proaching the suture. The increase in height is 0*45, the in- 

 crease in breadth ()•()") ; this Ammonite is altogether enve- 

 loped without an umbilicus. It is found with the Goniatites 

 of the limestone of Oberscheld near Dillenburg, and in the 

 Martenberg mine in the territory of Waldeck. The specimens 

 that I possess of the first locality are at the most one inch, and 

 otherwise altogether resemble those of Waldeck. This Am- 

 monite has nothing but the enveloped form in common with 

 A. Munsteri, which M. von Buch believed to be allied to A. 

 retrorsus ; on the contrary, it agrees so closely in the form and 

 lobes with A. simplex,von Buch, that perhaps they ought not to 

 be separated as distinct species. For the increase in height 

 M. von Buch gives for A. retrorsus only 0*32, and for A. sim- 

 plex 0*4 ; in all cases the height in both increases more rapidly 

 than the breadth, so much so that the thickness in the outer 

 whorls diminishes very fast, and the larger the Ammonite is 

 the more discoid is its form. In A. simplex, as M. Buch has 

 described and figured it, the dorsal lobe is considerably larger 

 and the lateral lobe smaller, and of the same depth as the dorsal 

 lobe. That is the only distinction between this Ammonite and 

 A. retrorsus. The name of retrorsus is given from the very 

 delicate striae seen on the shell. On the lateral surface the 

 terminations of the folds form a very smooth curve, inflected 

 behind : they ascend again afterwards towards the back, and 

 form above a narrow and deep sinus, the concavity of which 

 is directed towards the front. The breadth and depth of the 

 dorsal sinus appear to correspond to the breadth of the back 

 in the different species. The smaller the back the deeper and 

 more contracted is the sinus : in A. lateseptatus and A. Listeri 

 it is very large and flat. 



The A. retrorsus is the only species in this section that up 

 to the present time has been found in the schistose rocks of the 

 Rhine. A. simplex, which is nearly allied to it, and which is 

 said to be found at Rammelsberg near Goslar, comes perhaps 

 from the limestone of Grund, the fossils of which have a near 

 affinity to those of the transition limestone of the Eifel. To 

 these two Ammonites the A. ovatus, Munster, is intimately 

 allied. The description gives us no important difference either 



