174 GUIDE TO THE FOSSIL INVEETEBEATE ANIMALS. 



Gallery individuals, but was kept in check by the close coiling. 



TaWeease "^^^^^^^^^^ forms appeared first in the Upper Trias (e.g. Coclilo- 

 1^ ccras), but it was in the Cretaceous Epoch that they were 



Wall-ease first a large element in the fauna. Turrilites (Fig. 96 c) in 

 ^* the strict sense and various genera of similar form appear to 

 be derived from such antecedent forms as Cosmoceras and 

 Douvilleiceras, which they resemble in ornament. In some 

 of these the whorls are closer than in others. Helicoceras 

 and Hetcroccras (Fig 96 h) begin as asymmetrical spirals, but 

 turn off in a different direction in old age. They and some 

 of the Hamites are supposed to be connected with Acantho- 

 ccras. The direction of the turretted coil varies : in the 

 Senonian Hderoceras j^olyplocicm from Westphalia it is dextral, 

 as in most gastropods ; in the Turrilites of the Chalk 

 Marl 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 retrogressive, 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 previous 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 grows 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 itself. 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. Whatever 

 may be the affinities of certain straight-shelled Palaeozoic 

 cephalopods, the earliest fossils that show undoubted traces 

 Wall-case of the enclosino; mantle are Aulacoccras and Atractitcs of the 

 Upper Trias. These have guards, but the phragmocone is 

 relatively large, and in the former retains traces of longi- 

 tudinal ornament. 



