of the Chalk Formation. 383 
outstretched Baculite ; 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 plications (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 (aufgeblaht) that they do not 
pass over the back and cannot unite with each other from the 
two sides. The sipho then remains (bleibt zuriick) in a deep 
furrow. The secondary plications (Hiilfsrippen) 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, interruptus, 
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 
