THE JURASSIC PERIOD 



231 



ing' and retrogression in the next period, followed by extinction" 



(Chamberlin and Salisbury). The reader is again referred to the 



table on page 99 which outlines the evolution 



of the chambered-shell Cephalopods. 



We learned that the Dibranch Cephalopods 



made their first appearance in the Triassic 



period. In the Jurassic these forms were ex- 

 ceedingly abundant both in numbers of species 



and of individuals. Most characteristic of these 



were the Belemnites, so-called because of the 



long, conical, or dart-shaped, internal shells 



which are generally the only portions preserved 

 in the fossil state (see Fig. 142). 

 They were similar in appear- 

 ance to the modern Squids or 

 Cuttle-fishes. Some Jurassic 

 forms reached a length of over 

 two feet. A few specimens 

 from the English Oolite show 

 almost perfect preservation of 

 the original creature (see Fig. 

 143) . Ink-bags, like those found 

 in modern Squids, are some- 

 times so well preserved that 

 drawings of the fossils have 

 actually been made with ink 

 taken from their own ink-bags. 

 Arthropods. — Among the 

 Crustaceans the familiar Paleo- 

 zoic Trilobites and Eurypterids 

 were, of course, gone, and forms 

 much higher in structure had become abundant 

 and of pretty modern aspect. Thus, among Eu- 

 crustaceans, the long-tailed Decapods {Macru- 

 rans) or Lobster forms showed many genera and 

 species (Fig. 144), while the short-tailed Deca- 

 pods (Brachyura) or Crab forms made their first 

 appearance, though they were not numerous. Many types inter- 

 mediate between the long-tailed and short-tailed Decapods were 

 very common, these connecting forms being of special evolutionary 



Fig. 142 

 Internal shell of a 

 Belemnite, re- 

 stored. (From 

 Norton's "Ele- 

 ments of Geol- 

 ogy," by permis- 

 sion of Ginn and 

 Company, Pub- 

 lishers.) 



Fig. 143 

 A Jurassic Bel- 

 emnite, Belem- 

 noteuthis an- 

 tiqua. (Modi- 

 fied after Man- 

 tell.) 



