TEETIAET MAMMALIA. 173 



the type of existing orders, and the same may be said of the fishes, 

 principally teleosts. The change in the character of the inverte- 

 brate fauna is somewhat less marked than in the case of the verte- 

 brates; but yet certain important differences present themselves. 

 Thus, among the acephalous and gasteropod moUusks, by far the 

 greater number, in fact nearly all the types, are referable to exist- 

 ing families, and even in the oldest division, the Eocene, to exist- 

 ing genera, or to such as are very closely allied to them. Such 

 characteristic families as the Hippuritidse and Caprotinidse, among 

 the bivalves, have completely disappeared, and, if we except some 

 half-dozen or more species found in Australian Tertiary deposits, 

 the same may be said of the Trigoniadse, as well as of the Am- 

 monitidse* and Belemnitidse among cephalopods, about the most 

 distinctive of the invertebrate forms of the entire Mesozoic series. 

 Among the Tertiary invertebrates must be noted the extraordinary 

 development of the foraminiferal forms Nummulites and Orbitoides, 

 which, by their prodigious numbers, make up some of the most 

 stupendous deposits known to us. But that feature of the Ter- 

 tiary fauna which above all others arrests attention is constituted 

 by the class Mammalia. 



The most striking fact that presents itself in connection vrith 

 the history of these animals is their very sudden introduction, both 

 as to individual numbers and diversity of form, almost with the 

 beginning of the period, a circumstance of no little significance when 

 it is remembered that, in the period preceding, if we except the 

 doubtfully placed Meniscoessus, not even a trace of their existence 

 has been detected, and that all such forms as have been found in 

 the earlier Jurassic and Triassic deposits belong, as far as we are 

 able to determine, to the single order of the Marsupialia. In the 

 earliest division of the Tertiary, the Eocene, on the other hand, 

 we meet with the remains of individuals belonging to at least one- 

 half of all the recognised orders of the present day.f Thjis, we 

 have marsupials of the opossum type (Didelphis), insectivores, 

 rodents (as represented by the Sciuridse, or squirrels), cetaceans 



* A few ammonitio fragments have been found in the Tertiary deposits of 

 the Tejon group of California, and a Tertiary belemnite is claimed for Aus- 

 tralia. 



t The Ornithodelphia, Edentata, Probosoidea, Hyracoidea, and possibly 

 also the Sirenia and true Camivora, are still unknown. 



