of the Chalk Formation. 383 



outstretched Baculite j the Nautilus on the other hand arises 

 through a similar series of forms from the long-extended Ortho- 

 ceratites. 



Very many Ammonites of the chalk possess another peculiarity 

 by which they are easily recognized, and consequently also the 

 formation in which they are found. The phcations (Falten) or 

 ribs on the sides become always stronger and thicker the nearer 

 they approach the back ; in the Jura Ammonites they become 

 weaker from below upwards ; on the margin of the back itself 

 they are often so much expanded [avfgebldht) that they do not 

 pass over the back and cannot unite with each other from the 

 two sides. The sipho then remains (bleibt zurilck) in a deep 

 furrow. The secondary plications [Hulfsrippen) are of equal 

 strength with the chief plications, especially on the margin of 

 the back, but they disappear even on the middle of the side and 

 rarely unite with the chief plications. This gives the whole form 

 an unhandsome, almost clumsy aspect, which distinguishes it 

 very much from the handsome Jura Ammonites. Instances may 

 be taken from very different families : Am. Syriacus, interruptuSf 

 Rhotomagensis, denarius, varicosus, Sow. &c. This formation 

 evidently arises from the endeavour of the upper half of the ani- 

 mal to move quicker forwards on the back than the lower ventral 

 portion can follow. It is just this endeavour also that produces 

 the singular form of the Scaphites. On more close investigation 

 it would evidently be pronounced a disease of the Ammonite, 

 were not the form too constant, and did it not show too complete 

 an agreement in widely distant regions. It is universally seen in 

 all Scaphites, that they only then first forsake the regular form 

 of an Ammonite when the last septum is formed. The shell sud- 

 denly expands immoderately, the former law of the increase of 

 the whorls is entirely suppressed, the lateral ribs and promi- 

 nences are disproportionately separated from each other, new 

 ribs run down from the back and push themselves strongly for- 

 wards. The ventral side separates itself entirely from the whorl, 

 and the shell grows in a straight, no longer spiral direction ; yet 

 not long ; the expanded portion again contracts, the last chamber 

 appears as if drawn together, it anew curves round towards the 

 centre, but by this contraction the life of the animal is soon ter- 

 minated. If it is a disease of the Ammonites, then it is a real 

 epidemic which has attacked the Ammonites in the cretaceous 

 period, for the Scaphites are far from rare, of very various forms, 

 and almost peculiar to the middle cretaceous strata ; hence they 

 must be especially regarded as characteristic fossils for the cre- 

 taceous formations. And these deposits even on the Missouri 

 are no less marked by them than by the large Inocerami. For 

 one of the most beautiful Scaphites, the S. Nicolletii, which 



