PRINCIPLES OF ANIMAL lilOUXlY 71 



drawings, and the summary (directions for which are given below), 

 will answer most of them. 



1. Study an orthocone (Loxoceras or Orthoceras, for example). Thin 

 type of cephalopod was particularly common in the Ordovician and 

 Silurian periods. What Is the form of the shell ? The form of the sutures? 

 The specimen is usually only a fragment of the entire shell. Thus, in the 

 Museum of Geology at the University of Michigan is a fragment of an 

 orthocone 6J inches in diameter at its larger end, 4J^ inches in diame- 

 ter at its smaller end. This fragment is 18 inches long. If the piece were 

 completed at its smaller end, how long would it be? Since the animal 

 lived only in the undivided chamber at the larger end of the shell, the 

 shell was many times larger than its occupant. 



Modern cephalopods progress backward by means of the siphon. 

 (Examine the siphon of a squid and understand its operation.) How 

 would the long shell affect the animal's movements? 



Draw an orthocone, giving its name, to show the form of the shell and 

 of the sutures. A line drawing is sufficient but should be carefully made. 



2. Examine a gomphoceran (Poterioceras or some other). These 

 forms are recovered from the Ordovician to the Carboniferous periods. 

 What is the shape of the shell? Form of the sutures? How much of the 

 shell was occupied by the animal? Is this shell more cumbersome, or less 

 so than that of the orthocone? 



Draw a gomphoceran carefully (line drawing). 



3. In a nautiloid (Eutrephoceras is an example), what is the form of 

 the shell? Of the sutures? Examine a bisected fossil nautiloid, if 

 one is available, noting the form of the septa; compare it with the bisected 

 shell of the modern Nautilus. The nautiloids were most abundant in 

 Silurian and Devonian times, though some survived those periods, and 

 one of them, the pearly Nautilus, is still living. 



Draw a nautiloid, showing all the visible sutures. 



4. Compare the shell of a goniatite (Aganides or some other) with those 

 of the preceding forms, particularly the nautiloid. Note the form of the 

 sutures. Goniatites were most abundant in Carboniferous times (see 

 time scale). 



Draw a goniatite, being careful to represent all the sutures in their 

 correct form. 



5. Study a ceratite (Ceratites or any other). What is the form of 

 the suture? How do the sutures compare in complexity with those of a 

 goniatite? The ceratites were largely Triassic. 



Draw a ceratite to show its sutures. 



6. Study an ammonite (Scaphites or any other) . These reached their 

 climax in the Jurassic to the Cretaceous periods. The sutures are 

 very crooked fine lines on the surface. Do not confuse the coarse ridges 

 on the surface with them. Trace very carefully at least one suture 



