262 AN INTRODUCTION TO THE STUDY OF FOSSILS 



slight. At present they are represented only by the genus Nau- 

 tilus, which has given the name to the sub-order. 



Orthoceras (Fig. 114). Ordovician to Triassic. 



Shell usually straight in the form of a long, tapering cone 



(whence the name from Greek orthos, straight, + ceras, a horn). 



Surface smooth. Living chamber long. Sutures simple, 



straight or nearly so. Siphuncle usually 

 central. 



1. Sketch {a) side view; {h) top view. 

 Label sutures, living chamber if present, 

 siphuncle. 



2. How do you account for the succes- 

 sion of chambers and their equal spacing ? 



3. What was the function of the si- 

 phuncle ? 



4. What part of the body secreted the 

 shell ? 



5. What chamber did the body occupy 

 at the time of the animal's death ? Was 

 the siphuncle tilled or empty during life ? 



A 

 post. 



Fig. 114. — Thecephal- 

 opod, Orthoceras so- 

 ciale Hall, from the 

 marine Maquoketa 

 formation (Upper 

 Ordovician) of Iowa. 

 ( X 2-) il, an almost 

 complete internal 

 mold of the shell 

 showing the suture 

 lines and probably 

 most of the living 

 chamber, ant., an- 

 terior end ; l.ch., liv- 

 ing chamber; post., 

 posterior end ; su., 

 suture. B, an ideal 

 cross section showing 

 the siphuncle {si.). 

 {A, after Clarke.) 



Ryticeras (Fig. 115). Devonian. 



Shell varying from slightly curved to 

 loosely coiled. Siphuncle ventral, slightly 

 expanded between the septa. Surface 

 marked by coarse bands and at times by 

 spinous processes. 



I. Sketch side view, showing curve of 

 shell. 



Nautilus (Figs. 112, 113). 



Tertiary to present. 



Four living species of Nautilus are 



known ; these are from the Indian and 



Pacific Oceans. N. pompilius is described 



on page 251. 



