M2 PALEONTOLOGY 



another tendency that of reversion in old age to earlier 

 stages. Thus Meloceras elongatum tends to an orthocone 

 stage by lessening of curvature ; Ophidioceras reverts 

 abruptly to the same. In Palaeozoic times examples of 

 this are not very frequent, but among Mesozoic. Cephalo- 

 poda a tendency to uncoil is common : the next few 

 examples will illustrate this. 



Spiroceras calloviense (Fig. 41, c, c') is a common 

 fossil in the Kellaways Rock of Wiltshire. At first sight 

 it appears to be a gyrocone, but close examination shows 

 that it begins with a globular protoconch, around which 

 the tube-like shell winds closely for more than one 

 whorl, after which its curvature diminishes and it con- 

 tinues as an open (evolute) spiral. As this is a stage in 

 an uncoiling process it must not be called a gyrocone : the 

 term applied is criocone. 



Toxoceras (Fig. 41, /) is a genus of which various 

 species are known from Cretaceous rocks. It has the 

 form of a cyrtocone, and the protoconch and initial part 

 of the conch never seem to be preserved, so that direct 

 evidence of uncoiling is wanting ; but as it occurs so late 

 in time, and has sutures like those of contemporary coiled 

 and uncoiling forms, it is reasonable to regard it as a 

 stage in uncoiling (toxocone}. 



Baculites (Fig. 46, /, I'} is a long, straight, compressed 

 form, found in the uppermost Cretaceous strata. Its 

 initial stages are usually lost, but in specimens from 

 South Dakota they have been found to be substantially 

 similar to those of Spiroceras, the closely-coiled stage 



