of the Transition Formations of the Rhine. 159 



12. Ammonites iritumescens, n. s. PI. II. fig. 2, a, h,c. 



The dorsal lobe is rather less broad than deep^ the sides 

 ascending rapidly to the dorsal saddle. The elevation in the 

 middle reaches to at least half the height of the lobe. The 

 dorsal saddle has much greater height than breadth^ occupies 

 the middle of the side^ and declines rapidly towards the late- 

 ral lobe ; this is almost as deep as the dorsal lobe, is angulose, 

 and its ventral side raised, at first rapidly and afterwards 

 more gradually, towards the suture, almost to half the height 

 of the dorsal lobe. The increase in height is 0*455 the in- 

 crease in breadth 0*5. Of the five or six whorls the inner 

 ones are almost wholly enveloped, so that only a narrow and 

 deep umbilicus remains. This ammonite is found with the 

 two preceding at Sessacker, near Oberscheld. In its form it 

 bears some resemblance to A. carinatus\ it is, however, thicker 

 and less discoid, for with a similar increase in height it has 

 more rapid increase in breadth. The lobes moreover suffi- 

 ciently distinguish it. The greatest thickness is found towards 

 the suture, the sides decline somewhat rapidly towards the 

 back. The unchambered part is wanting in the specimen 

 represented. The complete ammonite must have a diameter 

 of at least four inches. 



13. Ammonites orbiculus, n. s. PI. I. fig. 12, a,b. 



The dorsal lobe is twice as broad as it is deep, and its sides 

 gradually ascend to the dorsal saddle. The middle elevation 

 reaches to half the height. The dorsal saddle is broad and 

 rounded, and occupies at least three-fourths of the side ; its 

 breadth is greater than its height. The lateral lobe is angu- 

 lose, and almost the same depth as the dorsal lobe ; its ventral 

 side wall ascends towards the suture, but only reaches to about 

 half the height of the dorsal saddle. The increase in height 

 is 0*47^ the increase in breadth 0*56. The inner whorls are 

 almost entirely enveloped, forming a narrow and deep umbi- 

 licus. 



This ammonite has been found in the state of pyrites in the 

 environs of Gerolstein. The only specimen that I am ac- 

 quainted with, now figured and described, is in the collection 

 of M. Zehler at Crefeld. It is peculiarly interesting, as the 



