164 INVERTEBRATE PALAEONTOLOGY 



tions, but failed to achieve, or maintain, enrolment, 

 This type of reversion culminated in the Cretaceous 

 Baculites, which, apart from internal details, was 

 essentially a secondary Bactriticone. Other groups, ex- 

 emplified by the Pulchelliidae of the Cretaceous period, 

 retained the " Ammoniticone" involution, but allowed 

 their septa to revert to a degree of complication hardly 

 greater than that of the " Goniatites." For example, the 

 suture-line of Tissotia, from the Cenomanian, is hardly 

 to be distinguished from that of Ceratites, from the 

 Muschelkalk. Although either of these regressive de- 

 velopments may have been associated with environmental 

 influence, their appearance seems constantly to have 

 preceded extinction of the groups affected. Whether 

 such groups perished in the Trias, or persisted until the 

 final collapse of the class in the Cretaceous period, they 

 usually displayed, in their latest members, reversion 

 towards ancestral features. The condition has been aptly 

 described as " second-childhood." It must be admitted, 

 however, that certain apparent exceptions to this rule 

 are known. For example, Indoceras, an Upper Cre- 

 taceous genus from India, is not merely an " Ammonite" 

 in shape, but has septa which in some particulars rival 

 those of the most elaborate Triassic types. The signifi- 

 cance of this and similar cases is open to doubt ; but, 

 in spite of exceptions, regression during degeneracy 

 seems a definite principle of evolution. 



Two common features of specialization, increase in 

 size and addition of shelly matter, are readily traceable 

 in fossil material. The former quality is not so marked 

 among Invertebrates as among Reptiles and Mammals, 

 especially in the catagenetic stages of its over-develop- 

 ment ; the latter is very clearly shown in a great variety 

 of groups. Both tendencies seem particularly liable 

 to persist beyond the stage in which they make for 



