DEPARTMENT OE GEOLOGY 
21 
vided into five equal rays. Echinohrissus, Pygaster, Hybocly- 
peus, and Clypeus represent the other type, in which the sym- 
metry is bilateral instead of radial. 
Many species of Brachiopods are shown but the majority of 
them belong to the genera Terebratula and Rhynchonella. 
Molluscs were the most characteristic group of invertebrate 
animals in Jurassic time and both the Pelecypods and Gastro- 
pods bore marked resemblance to modern forms. Among Pele- 
cypods are shown Oysters such as Ostrea and GryphcBa; Scal- 
lops such as Pecten, Lima, and Aviciila; and Mussels and Clams 
such as Modiola, Pinna, Pleuromya, Pholedomya, Trigonia and 
Cardina. Diceras is a shell in which the beaks of each valve are 
long and coiled like a horn. Nerinea, Natica, Pleurotomaria, 
Cerithiuni, Turbo, and Trochus represent the Gastropods of this 
period. 
Three distinct types of Cephalopods are shown. The first 
is illustrated by Nautilus, with a series showing variations in size 
and ornamentation. Polished sections show the internal char- 
acters of the shell. 
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 
t}^pical 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 Belemnifes 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 
Trachyteuthis, Geoteuthis and Plesioteiifhis, show the impres- 
sion of the bod}q others only the skeleton. 
A good series of Crustaceans of the type of the lobster and 
horse-shoe crab is shown. Among the Insects shown may be 
