174 GUIDE TO THE FOSSIL INVEETEBHATE ANIMALS. 
Gallery 
VII. 
Table-case 
15 . 
Wall-caEe 
3 . 
Wall-case 
8 . 
indivicliials, but was kej)t in check by the close coiling. 
Turretted forms appeared first in the Upper Trias (e.g. Cochlu- 
ccras), but it was in the Cretaceous Epocli that they were 
first a large element in the fauna. Turrilites (Fig. 90 c) in 
the strict sense and various genera of similar form appear to 
be derived from such antecedent forms as Cosmocems and 
Donvilleiccras, which they resemble in ornament. In some 
of these the whorls are closer than in others, lldicoceras 
and Hcferoccras (Fig. 90 h) begin as asymmetrical spirals, but 
turn off in a different direction in old age. They and some 
of the Flamites are supposed to be connected with Acantho- 
ccras. Tlie direction of the turretted coil varies : in the 
Senonian TIctcroceras 2)oly23l()cum from AVestphalia it is dextral, 
as in most gastropods ; in the Turrilites of the Chalk 
Alarl it is generally reversed or sinistral ; in those of the 
Gault it is indifferently dextral or sinistral in the same 
species. 
The various changes in these Cretaceous Ammonoidea 
may be described as retrogi’essive, for they are in some 
respects a going back along the line followed in the previous 
evolution of the Order. They were followed by complete 
extinction, for the Order did not persist into the Cainozoic 
Era. Were these changes in accord with changes in the 
environment, and was the extinction of the Order due to 
inability to keep pace with change of conditions ? Or were 
the changes inherent in the constitution of the ammonites, a 
necessary result of their jirevious history, and do they signify 
a true degeneration and decline, out of all accord with the 
surroundings ? One fact not yet mentioned may have a 
bearing on this problem. It is that in some shells the last 
bend grew in such a direction that in old age its aperture 
was brought up against a preceding part of the shell, so that 
the arms of the animal can scarcely have emerged ; by 
continuing its own growth, it seems that the individual 
killed itseli’. Did the race do the same ? 
Order COLEOIDEA or BELEMNOIDEA. — In modern 
times this has taken the place formerly occupied by the 
Ammonoidea and before that by the Nautiloidea. AVhatever 
may be the affinities of certain straight-shelled Palaeozoic 
cephalopods, the earliest fossils that show undoubted traces 
of the enclosing mantle are Aulacoccras and Airactites of the 
Upper Trias. These have guards, but the phraginocone is 
relatively large, and in the former retains traces of longi- 
tudinal ornament. 
