Fishes of the Western NortJi Atlantic 517 



lobe and with its axis continuing on the general longitudinal axis of the trunk (diphy- 

 cercal). A short separate anal fin marked off from caudal by a deep notch in some. 

 Pectoral and pelvic fins well developed, the latter situated about on a level with anus, 

 far rearward. Bases of fins thick and fleshy, their outer parts thin and flexible. 



Eyes large, on sides of head, encircled by a narrow free lid that is not movable; 

 orbits tending to come together above. Spiracle absent after birth but temporarily 

 represented during embryonic development. Nostrils large, close in front of mouth, 

 each connected with outer corner of mouth by a deep groove roofed over by lateral 

 lobe of upper lip, thus forming a closed channel through which water is drawn into 

 mouth in breathing; nasal aperture incompletely subdivided by a complex dermal fold, 

 projecting inward from outer side of nostril and supported by a cartilage. Mouth on 

 lower surface of head, a little in front of level of front of eyes, transverse, small. Lips 

 thick and fleshy and supported by complex cartilages; upper lip of three parts, a trans- 

 verse median lobe and a large lateral lobe on either side separated from the median part 

 of the lip by a groove connecting mouth with nostril. Roof of mouth without transverse 

 curtain or breathing valve; floor of mouth with a short and fleshy wedge-shaped tongue,* 

 free at tip, reaching forward between posterior parts of mandibular dental plates from 

 close in front of first internal gill cleft. External gill openings low down on sides, close 

 in front of origins of pectoral fins. Anus close in front of origin of pelvic fins, the uro- 

 genital openings close behind. 



Dental plates persistent, the base growing constantly;' basal part smooth, of 

 vasodentine with more or less conspicuous ridges or prominences (known as tritors) 

 covered with vitrodentine; two pairs, an anterior vomerine and a posterior (usually the 

 larger) palatine on roof of mouth; one mandibular pair on floor of mouth. Inner posterior 

 edges of palatines and of mandibulars more or less concealed by expansions of fleshy 

 tissues of roof and floor of mouth. 



Skin more or less roughened along midline of back by small denticles in some 

 species, also here and there along mucous canal system^" and on claspers and tena- 

 cula of males; otherwise naked and slippery with mucus. 



Sensory mucous canal system well developed, especially on head, much more 

 noticeable than in other cartilaginous fishes. Lateral canal continuous from below eye 

 rearward onto axis of caudal, a closed canal in some but an open groove in most, its 

 wall supported by numerous crescent-shaped limy bars supposed to represent modified 

 dermal denticles; mucous canals on head partially closed, and marked by series of large 

 pores ; each side of head with a cranial canal running forward above eyes out onto snout, 

 the two connected across nape in front of dorsal spine by an aural canal; cranial canal 

 continuing downward from its junction with aural canal as the occipital canal to junction 

 with lateral canal, then turning forward below eye to snout, giving off first a jugular 

 branch to region of gill opening, then either an oral branch and an angular branch 



8. Sharks also have a distinct tongue, but Skates and Rays do not. 



9. They do not show any sign in their embrj-onic development of being formed by the fusion of separate teeth or 

 denticles, but they are usually regarded as homologous with the teeth of elasmobranchs. 



10. In some fossil chimaeroids the skin was covered with strong denticles while others had plates on the head. 



