DEPARTMENT OF GEOLOGY 27 
etc. These are free-moving forms in contrast to the stemmed 
crinoids so abundant in earlier times. 
Pelecypods and Gastropods are illustrated by many speci- 
mens, mostly of modern types, such as Ostrea, Exogyra, and 
Gryphosa, which were Oysters of many and curious shapes. 
Pectens, Vola, etc., were Scallops. 
, Among bivalve molluscs the family of Rudistae is unique, 
and characteristic of this period. In shells of this family one 
valve is enormously enlarged, and somewhat funnel-shaped; 
the other valve is small and acts as a lid. Hippurites, Spheru- 
lites and Radiolites belong to this family. Inoceramus sometimes 
reached enormous size. 
Among Cephalopods are shown specimens of Nautilus of 
modern types; also members of the Ammonite family, which 
took on various and intricate forms. The series of P lac entic eras, 
an ammonite with coiled shells often two feet in diameter, is es- 
pecially worthy of note. The specimens are chiefly from the 
Bad Lands of South Dakota. Many varieties of shape are 
found among the Ammonites, from straight shells to hook-shaped, 
partly uncoiled, spirals, etc. The genera are often named from 
their characteristic forms, some of them being as follows: 
BacuUtes, rod-shaped; Hamites, hook-shaped; Helicoceras, an 
open spiral; Scaphites, boat-shaped; Turrilites, tower-shaped. 
Cenozoic Era; Age of Mammals. — With the close of the 
Mesozoic era the many and varied forms of reptilian life dis- 
appeared. The mammals which had existed as an unimportant 
form during nearly the whole of Mesozoic time now became the 
predominant type. Fossils from this era are exhibited in the 
west half of Hall 59, and Halls 60 and 61. 
Tertiary Period. Tertiary plants and invertebrate fossils 
shown include Flahellaria, leaves of a palm which grew in the 
Eocene epoch near Green River, Wyoming. Also, leaves of 
Acer, or maple, and other trees of modern genera. 
Such remains, with others that are found, indicate that a 
subtropical climate, like that of Florida, prevailed at this period 
over the northern United States. Even so far north as Green- 
land, the climate was so mild that cypress and cedar trees grew 
in profusion. 
Nummulites. — These are abundant and characteristic fossils 
of this period. They are shells of a Rhizopod, which in Europe 
and Africa formed limestones many thousand feet in thickness. 
