68 



THE ELASMOBRANCH FISHES 



of these arches is made up of two segments, one dorsal, the other ventral in 

 position. 



Three labial cartilages are usually present on a side in sharks, two dorsal, 

 the so-called premaxillary and the maxillary cartilages on the palatoquadrate, 

 and one ventral on the mandible. Of the dorsal segments the anterior is the 

 shorter. When well developed the labials serve to reduce the gape of the mouth. 



Fig. 74. Extrabranchial cartilages of Baia erinacea. (From Foote.) A. Dorsal view. 

 B. Ventral view. 



ex.l)., extrabranchial cartilage; ex.h., extrahyoid cartilage; md., mandible; mp., median 

 piece; p-o., palatoquadrate (pterygoquadrate) cartilage. 



An interesting series may be followed in the specialization of these carti- 

 lages from the simple condition of a single cartilage to the tripartite condition 

 just mentioned. Heptanchus cinereus has a single labial located dorsally, 

 which because of its small size long escaped observation (Fiirbringer, 1903). 

 In Heptanchus maculatus (1., fig. 48) there is a single cartilage, shaped like a 

 tuning fork, which extends across the gape of the mouth and ends dorsally in 

 two horns. It gives no evidence, however, of segmentation. In Hexanchus, the 

 labial extends across the gape of the mouth and according to Gegenbaur 

 (1872) is more or less divided into an anterior and a posterior division. In 

 the notidanids, then, we have practically a complete transition from the single 

 rudiment to the condition found in more specialized forms. 



It may be said of transitional rays that the labial cartilages are poorly 

 developed (Rhinohatis) . 



