i6 FIELD MUSEUM OF NATURAL HISTORY 
and body cavity of an ancient, huge Rhizopod. But most 
paleontologists regard the masses as of wholly inorganic 
origin. 
Plants are represented impressions of seaweeds such as 
Paleophycus and other Algae. Trails and borings of marine 
worms such as Scolithus and Arenicolitcs are shown. Lingu- 
Icpis is one of the earliest Bradiiopods. The tracks of Climac- 
tichnites on a large slab of sandstone from Wisconsin were 
probably made by some large mollusc, although they have been 
considered by some to be Trilobite tracks. Trilobites are the 
most characteristic fossils of this period and a good series of 
specimens is shown, representing such genera as Ogygopsis, Para- 
doxides, Olenellus, Chonocep halites, Dikelocephalus, Ptychoparia, 
Agnostus, etc. These illustrate range in size. 
Ordovtcian Period. — The fossils of this period, as a general 
rule, are well preserved and abundant. Nearly all classes of 
marine invertebrate animals are represented in the collection 
and the abundance of life is shown by slabs of Plectambonites , 
Leptmia, etc., -which are literally masses of shells. 
Receptaculites and Brachiospongia are calcareous sponges. 
Graptolithus, Diplograptus, Tetragraptus, etc., are Hydroids 
known as Graptolites, abundant fossils of this era. The name 
is derived from the Greek word meaning “to write,” and refers 
to the plume-like nature of the remains. 
Corals were rare in this period. Occurring forms are repre- 
sented by Streptelasma, a cup coral and Columnaria, in large 
masses of honeycomb coral. 
Glyptocrinus and locrinus, specimens of which are shown, 
are among the earliest known crinoids. They were attached by 
a jointed stem and bore many-branching arms. They have 
been appropriately termed sea -lilies. 
Bryozoans are among the most important and abundant fossils 
of this period. They are illustrated by Bythopora, Callopora, 
Batostoma, Constellaria, Monticulipora, etc. 
Brachiopods, though found only in small numbers at the 
present day, were in Paleozoic times one of the most abundant 
and characteristic forms of marine life. In structure they have 
points of alliance with the Worms on the one hand and with 
Molluscs on the other. 
Articulate Brachiopods, or those having a well-developed 
hinge, are represented by Strophomena and Rafinesquina of the 
