348 THE PINEAL ORGAN 



first that great class of extinct animals the trilobites (Figs. 97, 100, 102, 

 Chap. 11). These vary greatly in size and form, and they range from the 

 Cambrian to the late Carboniferous and Permian eras. According to 

 recent estimates of geologists the Cambrian age comprises a period from 

 500,000,000 to 750,000,000 years ago, and the Permian, which marks the 



Fig. 245. — Attempted Restoration of Brain of Kiaeraspis in dorsal view. 

 (After Stensio.) Compare with Fig. 22, Chapter 3, p. 28. 



die. : diencephalon. mec. : 



ep. : epiphysis. me d. . 



f. rh. : fossa rhomboidalis. met. : 



hab. I. : left habenular ganglion. olf. c. 



hab. r. : right habenular ganglion. sp. d. 



hy. s. : hypophyseal sac. sp. v. 



mesencephalon. 



medulla. 



metencephalon. 

 : olfactory capsule. 

 : dorsal roots of spinal nerves, 

 ventral roots of spinal nerves. 



lat. : roots of the prootic lateral nerves. /. to X. : cranial nerves 1 to 10. 



end of the Palaeozoic period, from 215,000,000 to 280,000,000 years ago. 

 When the first trilobites existed other invertebrate phyla were abundant, 

 and it is considered that the trilobites and kindred types were preceded 

 by a hypothetical class for which the name Protostraca has been suggested, 

 the term signifying the earliest type of animals possessing a shell — the 



