31 
because made up of hexagonal parallel columns, Orthis — a genus 
of the class of Brachiopods, characteristic of this epoch. 
Brachiopods are sometimes called lamp shells, on account of 
their resemblance to a Roman lamp; the two valves of the shell 
are unequal in size, and the beak of the larger curls over on that 
of the smaller. Though found only in small numbers at the pres- 
ent day, they were in Silurian times the most abundant and char- 
acteristic form of marine life. In structure they have points of 
alliance with the Worms on the one hand and with Mollusks on 
the other. Pleitrotoinaria, Murchisonia — Gasteropod or univalve 
Mollusks common in the Trenton epoch. 
Case 2B. — Hudson River and Medina epochs. Brachiopods 
are represented by large slabs containing Leptoe7ia, also many 
species of Lingiillela^ Lmgitla and Rhynchonella. Species of the 
two latter genera exist at the present day and may be seen in 
Hall 25, Department of Zoology. They afford a remarkable ex- 
ample of the power of a genus to survive the vicissitudes of time. 
Glyptocrinus^ locrinus, — Crinoids, the class of Echinoderms most 
abundant in early times. Attached by a jointed stem and bearing 
many branching arms, they have been appropriately termed sea- 
lilies. 
Arthrophycus — supposed to represent the leathery stems of 
seaweeds. Some authorities, however, regard this fossil as repre- 
senting the markings of worms. 
Case 2C. — Hudson River and Niagara epochs. Eridophyl- 
lum. — Corals of the Cyathophylloida^ or cup-coral family. Haly- 
sites. — Corals of the Halysitidae or chain-coral family. Nearly all 
Silurian corals belong to these two or the Favositid family. 
Streptorhynchus. — A representative Brachiopod. Orthoceras 
—many specimens illustrating the size and distribution of this 
Silurian Cephalopod. Above Case D is a drawing showing how 
the animal lived in its shell. It occupied only the outer chamber, 
but was connected with the other sections by a large siphuncle. 
Cases 2D and E. — Niagara and Lower Helderberg periods. 
FavositeSy Halysites — Corals. Eucalyptocrmus — Crinoids. IllcE7^^ls 
— Trilobites. Bythotrephis — probably marine algae. Pentameriis — 
a large and abundant Brachiopod, characteristic of the Niagara 
beds of the Mississippi Basin. Spirifer, Rhynchonella — other com- 
mon Brachiopods. Eurypterus — a large number of specimens 
