24 FIELD MUSEUM OF NATURAL HISTORY 
The second type, the Ammonites, is the largest and most 
characteristic group of Jurassic invertebrate fossils. They are 
distinguished by their complex suture lines. A large and repre- 
sentative collection of this group is shown, comprising all the 
typical genera, and illustrating the variations in size from Har- 
poceras, less than half an inch in diameter, to Olcostephanus, 
attaining a diameter of three feet. Many of these specimens 
have the sutures colored to bring out their outlines. Polished 
sections show the interior of the shells. 
The third type is that of the squids. These had no external 
shell but an internal bone or guard. A model shows the form of 
the animal and the location of the guard. Usually, all that is 
found fossil in Belemnites is the guard, of which quite a number 
are shown, but in several of the specimens the ink sac and hooks 
which are attached to the arms can be seen. Some of the speci- 
mens from the Lithographic limestone of Solenhofen, such as 
T Tachytcuthis , Gcotcuthis and Plesioteuthis, show the impres- 
sion of the body, others only the skeleton. 
A good series of Crustaceans of the type of the lobster and 
Horseshoe crab is shown. Among the Insects shown may be 
mentioned jEschna and Petalia, Dragonflies; Belostomum, an 
ant; Nepa, a water bug. 
Fishes of the Jurassic are represented (Hall 36) by a large 
number of specimens from England, Bavaria, and Wurtemburg. 
These are chiefly of two types. The Ganoids, which were the 
predominant type, may be recognized by their heavy, angular 
scales. The following genera are well represented: Caturus, 
Aspidorhynchus, T hr is sops, Leptolepis and Dapedius. The 
Selachians or sharks, having neither bony skeleton nor scales, 
are represented by the teeth and fin-rays only. Some genera 
of this type shown are Acrodus, Hybodus, and Chondrosteus. 
Icthyosaurs, from the Jurassic of England and Germany are 
represented (Hall 36) by an unusually fine series of skulls and 
skeletons. The Icthyosaurs, or fish-lizards were a type of marine 
reptile having a fish-like body and tail, a short neck, a long, 
pointed snout, enormous eyes and paddles somewhat like those 
of a seal. They attained a length of twenty-five to thirty feet. 
A model exhibited on the floors shows something of their out- 
ward appearance. 
A second form of marine reptile common to this period is that 
of Plesiosaurs. These were more slender and graceful animals 
