88 VERTEBRATE SKELETON 



Labial cartilages occur in most sharks, though lacking in some and 

 reduced in skates. Nothing need be added to the statements on 

 p. 68. Meckel's cartilage (lower jaw) calls for but shght notice. 

 It is articulated to the hinder end of the pterygoquadrate and is 

 capable of motion through a large arc in opening and closing the 

 mouth. 



In correlation with the great development of the pectoral tins and 

 their connexion with the tip of the head, the cranium of Raise has 

 several modifications. Some have the rostrum enormously devel- 

 oped (fig. 88), and in sawfishes (Pristidae) it extends far forwards, 

 its lateral margins bearing denticles or larger teeth. In skates the 



Fig. 91. — Skulls of (A) embryo (45 mm. long) Chimce.ra (Dean, '06) and B, 

 adult Chimcera (Allis, '17). ch, ceratohyal; dp, dental plates; Ic, labial cartilages; hm 

 hyomandibular; m, Meckelian; n, nasal capsule; pq, pterygoquadrate. 



anterior angle of the pectoral fin is connected with the head by an 

 axial process, arising in line with the propterygium, and usually 

 articulated with the preorbital crest. In Trygon, where the rostrum 

 is short, the axial processes of the two sides extend beyond the head. 

 Torpedo has two rostral bars, the middle of the usual three hav- 

 ing been lost. Most skates have lost the primary flexure of the 

 head, and lack basal angle and dorsum sellas (p. 70), and the hypo- 

 physial fossa has been obliterated. Labial cartilages are poorly 

 developed in skates. 



HoLOCEPHALA. — Thcsc sharks (often called raffish) differ from 

 other Elasmobranchs in being autostylic. They have three rostral 

 cartilages (fig. 92), the middle being at a higher level than the others. 

 The short, strong jaws support the large dental plates. No hyo- 

 mandibula is evident in the adult, but it is not decided whether it be 

 fused with the otic region or absorbed in the quadrate part of the 

 pterygoquadrate. The rest of the hyoid arch is pecuhar in having 



