168 GEOLOGICAL DISTRIBUTION, 



would be barely possible that an interchange between the faunal 

 and floral characters of that land-mass and Australia should not 

 have been effected to a greater extent than is indicated by the 

 isolated position, especially of the fauna, now existing. 



Cretaceous Fauna, The Cretaceous period presents in many 

 respects a marked contrast in its faunal characters to the period 

 preceding. While many of the most important or characteristic 

 of the Jurassic invertebrate types still persist, in many cases with 

 undiminished or even increased force, as, for example, the differ- 

 ent classes of the Mollusca, we meet here with a development 

 of other animal groups which in most of the periods preceding 

 were of comparatively insignificant import. Such are the Fora- 

 rninifera, which in their various forms (Globigerina, Rotalia, Tex- 

 tularia, Cristellaria) build up by their remains the great mass of 

 the chalk rocks, whose enormous extension is one of the most im- 

 posing monuments presented to the geologist. The sponges (Si- 

 phoDia, Jerea, Thecosiphonia, Ventriculites) here likewise find their 

 greatest development, some of the forms having their analogues in 

 the types that still inhabit the oceanic depths ; and the same has 

 been shown to be the case with the Cretaceous urchins (Echinoidea), 

 which are represented in great multitude and variety Cidaris, 

 Ananchytes, Galerites, Micraster, Discoidea. The corals are in 

 comparison feebly developed, and can by no means claim that im- 

 portance which they obtained in the Jurassic period. The Belem- 

 nitidse (Belemuites, Belemnitella) and Ammonitidce still constitute 

 the most important of the cephalopod types, the latter especially 

 presenting a very considerable number of characteristic forms, the 

 so-called unrolled ammonites Crioceras (with Ancyloceras and 

 Toxoceras), Hamites (Ptychoceras), Scaphites, Turrilites, Helico- 

 ceras, and Baculites. 



The bivalve and univalve faunas, while largely made up of 

 Jurassic types, show a marked advance over the corresponding 

 faunas of the period preceding in the much greater development of 

 the siphonate forms. The Sinuata among the former, which, if we 

 except the very abundant family of the Pholadomyidse, had hitherto 

 but scattered representatives, now acquire considerable importance, 

 especially in the families Veneridce, Tellinidse, Glycirneridse, Ana- 

 tinidse, Mactridoe, and Myidse. Among the non-sinuate forms 

 the members of the oyster family (Ostrea, Exogyra, Gryphsea) and 



