72 LABORATORY DIRECTIONS IN 



completely around one coil of the shell before attempting a drawing. The 

 chances for error are large, because adjoining sutures approach one 

 another very closely at various points. Examine other ammonites if 

 possible. 



Draw an ammonite at least natural size, showing two of the sutures. 

 The latter should be very accurate pictures of the specimen used, not 

 merely a diagrammatic representation of the kind of sutures found in 

 ammonites in general. 



B. EVOLUTION OF THE HORSE 



The development of the horse, as far as known from fossils, took 

 place entirely in Tertiary time. The undiscovered ancestor was undoubt- 

 edly a small animal, with five toes on each foot, and nails instead of hoofs. 

 In the following exercise the evolution of the horse will be traced with 

 respect to (1) number of toes, (2) form of teeth, and (3) size of body and 

 skull. 



Many of the specimens used in the laboratory are casts of fossils, 

 and must be handled with care. The student should see some of the 

 actual fossils also, if these are available. 1 



The Feet. 



1. Examine casts of the bones of the fore and hind feet of Eohippus 

 or Orohippus. They are of natural size. How many digits in each foot? 

 Are any of the digits distinctly shorter than the rest? 



From the wall chart note the geological age to which these forms be- 

 long. Draw both fore and hind foot, representing the proportions with 

 care, and carefully distinguishing the individual bones. Indicate by 

 Roman numerals which of the ancestral five digits are left (see Shull, 

 LaRue and Ruthven, " Principles of Animal Biology," Chapter XV). 

 The figure may be less than natural size. 



2. Study a cast of the foot of Mesohippus. How many digits? 

 Which ones? What is the relative size of the various digits? Compare 

 in size with Eohippus or Orohippus. 



To what geological period does Mrsohippus belong? Draw tin- 

 foot of Mesohippus with care, indicating which digits are present. 



3. Examine the fore foot of Hypohippus. Compare in size with the 

 foot of Mesohippus. Note the size of the third digit as compared with 

 the second and fourth. Is the third digit relatively larger, or relatively 

 smaller, than in Mesohippus? Did the lateral digits of Hypohippus 

 reach the ground? Observe the nodules at the back of the metacarpals 



1 Other genera of similar nature may l>e substituted for the ones named below. 

 Hypohippus ;uid Hipparion, which appear not to be in the direct line of evolution, may 

 be omitted if desired. 



