MESOZOIC ERA: THE AGE OF REPTILES 



531 



tites (Fig. 434 A, B) ; the complicated ammonite sutures beginning 



when the whorl is only two or three millimeters in diameter. In 



other words, each individual ammonite recapitulates the history of 



its race. It is consequently possible by studying 



a well-preserved individual to tell what its 



genealogical tree was. In no other animal can 



the evolution of the race be so well studied. It 



should be borne in mind, however, that the 



record of some of the stages of development is 



often omitted by " acceleration. " 



Since many of the species had a very short 

 life, they are especially important in showing 

 that widely separated strata are of the same age. 

 It should be remembered, in this connection, that 

 ammonites were free-swimming or crawling ani- 

 mals, and also that upon their death the gases of 

 purification caused them to float. They were 

 consequently moved, either by their own volition 

 or by the ocean currents after their death, over 

 wide areas, and hence are excellent " index 

 fossils." The ammonites contributed largely to 

 the Jurassic limestones, but of all this great 

 horde of shelled cephalopods, the simple-sutured 

 nautilus alone survived the Mesozoic. 



Naked Cephalopods {Belemnites). — In Jurassic 

 deposits straight, cigar-shaped fossils (Fig. 495) 

 are sometimes found in great abundance. These 

 belemnites are usually 3 to 5 inches in length, 

 although some specimens several feet long have 

 been found. Ink bags are associated in some 

 specimens, showing that their possessors darkened 

 the water to escape their enemies, as do the 

 squids of the present. These are the internal 

 shells of naked cephalopods which resembled the 

 squids of to-day in general appearance. This 

 class is first known from the Triassic, but once 

 started, the race rapidly increased, culminating in 

 the Jurassic and declining rapidly in the Cretaceous. Only one surviv- 

 ing genus (Spirula) is living to-day. The solid internal shells of the 

 belemnites constitute a considerable part of some Jurassic limestones. 



Fig. 495. — Res- 

 toration of a Belem- 

 nite. The solid, 

 cigar-shaped "guard" 

 (lower end) is a com- 

 mon fossil in the Juras- 

 sic and Cretaceous. 

 Next above the 

 "guard" is the phrag- 

 mocone, and above 

 this the proostracum. 



