14 FIELD MUSEUM OF NATURAL HISTORY 
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 
concavo-convex forms, and by Orthis, Rhynchotrema, etc., of the 
globose forms. 
Byssonychia, Vanuxemia, Cystodonta, Orthodesma, etc., are 
Pelecypods or bivalve molluscs. 
Bellerophon, Maclurea, Pleurotomaria, etc., are Gastropods 
or single-valved molluscs. 
Tentaculites are minute molluscs of the class of Pteropods. 
Conularia was perhaps also a Pteropod. 
Cephalopods were the largest known animals of this period. 
Orfhoceras, Camaroceras, Endoceras, etc., represent them. Some 
probably attained a length of fifteen or more feet when com- 
plete. 
Trilobites were, perhaps, the most characteristic fossils of 
early Paleozoic time. They were crustaceans having a horny 
shell similar to the crabs of the present day. They reached 
their greatest development as to number of genera in this period. 
Asaphus, Acidaspis, Illcenus, Dalmanites, Triarthurus, Trinu- 
cleus, etc., are typical genera shown. 
Scales and teeth indicate the presence of fishes, which were 
the only vertebrate animals known. The number of important 
animal types having existence in even the early geological periods 
is worthy of note. 
Silurian Period or Age of Molluscs. — Silurian fossils show 
continued development of life. Plants are still seaweeds, such 
as Bythotrephis and Arthrophycus. Some writers consider the 
latter to be casts of the trails of some animal. Astrceospongia, 
Astylospongia, Paleomanon and Cerionites are typical Silurian 
Sponges. 
Monograptus, Graptolithus and Diplograptus continue from 
the Ordovician but disappear, as do all Graptolites, with the 
close of this period. Stromatopora, one of the reef-building 
hydroids, although quite different from the Graptolites in habit, 
is referred to the same group. True corals were abundant and 
are fully illustrated in the collections. The reef-builders, such as 
Halysites or Chain coral, Favosites or Honeycomb coral. Theca, 
Heliolites or Sun coral, etc., predominated. Cup corals, as Zaph- 
