DEPARTMENT OF GEOLOGY 17 
concavo-convex forms, and by Orthis, Rhynchotrema, etc., of 
the globose forms. 
Byssonychia, V anuxemia, Cystodonta, Orthodesmay etc., are 
Pelecypods or bivalve molluscs. 
Bellerophofiy Maclurea, Pleurotomaria, etc., are Gastropods 
or single -valved molluscs. 
Tentaculites were minute molluscs of the class Pteropods. 
Conularia — perhaps also a Pteropod. 
Cephalopods were the largest known animals of this period. 
Orthoceras, Camaroceras, EndoceraSy etc. represent them. One 
specimen of the last named genus exhibited over the wall cases 
probably attained a length of fifteen or more feet when complete. 
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. 
AsaphuSy Acidaspisy IllcBnuSy DalmaniteSy TriarthuruSy Trinu- 
cleus y etc., are typical genera. 
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 the cast of the trails of some animal. Astreospongioy 
Astylospongioy Paleomanon and Cerionites are typical Silurian 
Sponges. 
MonograptuSy Graptolithus and Diplograptus continue from 
the Ordovician but disappear, as do all Graptolites, with the 
close of this period. Stromatoporay 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 
Haly sites or Chain coral, Favosites or Honeycomb coral, ThecOy 
Heliolites or Sun coral, etc., predominated. Cup corals, as Zaph- 
rentisy StreptelasmOy Omphyma, Eridophyllunty etc., are also shown. 
Crinoids and Crystoids showed a wonderful development both 
as to size and number in this period. Eucalyptocrinus, Perie- 
chocrinuSy SiphonocrinuSy Callicrinus, CaryocrinuSy Holocystites, 
etc., are some of the prominent genera represented. CladoporOy 
