56 



THE VERTEBRATE SKELETON. 



SPINAL COLUMN. 



In Myxinoids and larval lampreys, the notochord is en- 

 closed in a thick chordal sheath, in connection with which in 

 the tail region there occur cartilaginous pieces forming neural 

 arch elements. In the trunk region, however, no cartilage 

 occurs in connection with the spinal column, the only cartilage 

 present being that forming the radialia of the dorsal fin. On 

 the other hand in most species of lamprey (Petromyzon) 

 cartilaginous pieces forming imperfect neural arches (fig. 4, 

 B, 13) are found lying in the tough skeletogenous layer dorsal 



FIG. 4. A, DORSAL ; B, LATERAL AND C, VENTRAL VIEW OF THE 

 SKULL or Petromyzon marinus x 1 (after PARKER). 



1. horny teeth. 8. 



2. labial cartilage. 9. 



3. anterior dorsal cartilage. 10. 



4. posterior dorsal cartilage. 11. 



5. nasal capsule. 



6. auditory capsule. 12. 



7. dorsal portion of trabeculae. 13. 



lateral distal mandibular. 

 lingual cartilage, 

 branchial basket, 

 cartilaginous cup supporting 



pericardium, 

 sheath of notochord. 

 neural plate. 



to the notochord, and extending throughout the whole length 

 of the trunk and tail. Two of these pieces, which are probably 

 homologous with the neural plates (see p. 72) of Elasmobranchs, 

 occur to each neuromere, or segment as determined by the 



