64 



COMPARATIVE ANATOMY. 



The skull of Elasmobranchs presents the simplest conditions 

 and most easily comprehensible relations, so that it may be taken 

 as the starting-point for the study of the skull of all other 

 Vertebrates. It consists of a simple cartilaginous and fibrous 



FIG. 49. SKTTLL AND BRANCHIAL BASKET OF Petromyzon planeri. 



Lb, labial cartilage ; R, cartilaginous ring-shaped support of the snctorial mouth : 

 A, B, C, three other supporting plates of the suctorial mouth; ZB, lingual 

 cartilage ; Na, external nostril ; N, nasal sac ; Tr, trabeculae ; PQ, palato- 

 quadrate ; SS, fibrous cranial tube, which is cut through behind at MG (medul- 

 lary canal) ; OB, auditory capsule ; Hy, hyoid ; Ko, gill-openings ; t, posterior 

 (pericardial) cartilage of the branchial basket ; *,*, transverse bars of the branchial 

 basket ; C, notochord. 



capsule either immovably united with the vertebral column 

 (Squalidse) or connected with it by articulations (Rays and 

 Chimserse). 



FIG. 50. SKULL OF Heptanchus. 



WS, vertebral column ; GK, auditory capsule ; PF, AF, postorbital and ant- 

 'orbital processes ; Orb, orbit ; R, rostrum ; NK, nasal capsule ; t, region of 

 articulation of the palatoquadrate (PQ) with the skull ; G, articulation of lower 

 jaw ; Md, mandible ; Z, teeth. 



True bones are never developed, the cartilage being merely 

 calcified ; the palatoquadrate and the lower jaw are nevertheless 

 richly provided with teeth (Fig. 50, Z). 



