61 COMPARATIVE ANATOMY. 



The skull of Elasmobranchs presents the simplest conditions 

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

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

 Vertebrates. It consists of a simple cartilaginous and fibrous 



Tr ** OB 01 



KO 



FIG. 49. SKULL AND BRANCHIAL BASKET OF Pctromyzon planeri. 



Lb, labial cartilage : /.', cartilaginous ring-shaped support of the svctorial moutli : 

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

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

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

 lary i\inal) ; OB, auditory capsule ; Hy, hyoid : Ko, gill-openings ; t, posterior 

 (pericardia!) 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 

 Chimserae). 



MtZ 



FIG. 50. SKULL OF HcptancJivs. 



Ji'S, vertebral column; GK, auditory capsule; PF, AF, postorbital and ant- 

 orbital processes; Orb, orbit; R, rostrum; NK, nasal capsule; +, region of 

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

 jaw : Mil, mandible ; Z, teeth. 



True bones are never developed, the cartilage being merely 

 calcified; tin- |i:ila.t<!|ii:i(lrate and the lower jaw arc nevertheless 

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



