268 AN INTRODUCTION TO THE STUDY OF FOSSILS 



1. Sketch (a) the elongate flattened surface of a specimen 

 showing the sutures ; (b) top view ; label sutures, lobes, saddles, 

 umbilicus, septum, siphuncle. 



2. These fossils usually occur with mud filling all the chambers. 

 How did the mud penetrate the chambers ? 



3. Distinguish between septa and sutures. 



4. What does a single perfect shell of BacuUtes suggest as to 

 its ancestry ? 



Fig. 120. — Bacidites compress us Say, from the Cretaceous of South Dakota, i. A 

 young shell preserving the early coils. The living chamber occupied about one 

 half the length of the shell. 2. A specimen of same size as the last but with the 

 outer shell broken away revealing the sutures, — the outer edges of the septa. 

 Note the variation in the sutures from the apex of shell 2 to its wide end and on 

 through the progressively larger shells as shown in 3, 4, 5, and 6. 6 represents 

 the condition of the suture in the adult shell. (After Brown.) 



Order 2, Dihranchiata 



Cephalopods with shell internal or wanting, or exceptionally 

 external but not chambered (Argonauta). Head bearing eight 

 or ten arms around the mouth ; these are furnished with suckers. 

 The eyes are very large and of complex structure. Two gills 

 and two kidneys are present. An ink sac discharges its contents 

 through the funnel when the animal is startled, thus constituting 



