188 THE PLAGIOSTOMIA. 



fig. 2-3, the molars retain the keel; in old ones the keel would be less noticeable. 

 Anterior gill openings more than twice as wide as the hindmost, latter width 

 equals length of eye, three above the pectoral. Dorsals moderate, height about 

 equal length of base, upper angles bluntly rounded, hind margins convex, lower 

 angles not produced; origin of first dorsal behind the end of the base of the 

 pectoral, distance between the bases one and two thirds times the length of the 

 base of the first. Anal much smaller than the second dorsal, tip reaching the 

 base of the caudal, base distant nearly two thirds of its length from the caudal, 

 hind margin oblique, tip rounded. Subcaudal moderate in depth, with a slight 

 lobe. Pectorals short, broad, angles rounded, hind margin nearly straight. 

 Ventrals broad, hind margin slightly oblique. Scales small, with median keel, 

 larger, with scattered small tubercles on the back, smoother below. Back 

 rusty brown, yellow below, with scattered spots of black, from mere dots to 

 spots as large as the orbit or larger, over the entire body and fins. Commonly 

 the spots show a tendency toward grouping in twos or fours; in cases they are 

 more confluent. On some there are five or six rather indefinite transverse bands 

 of darker separated by spaces of equal width : a band crosses the nape, another 

 lies in front and a third behind the first dorsal, one in front and one behind the 

 second dorsal and one in front of the caudal. A darker area extends from each 

 orbit across each cheek. 

 Galapagos Islands. 



Squalidae. 



Body subround; tail slender, compressed. Head depressed; rostral car- 

 tilage a simple trough-shaped process of the skull. Nostrils remote from the 

 mouth. Mouth not greatly arched, with labial folds and a deep groove at each 

 angle, slightly protrusible, upper jaws loosely articulated to the skull. No 

 nictitating membrane. Spiracles present. Five gill clefts in front of each 

 pectoral. Two dorsal fins, each preceded by a spine. No anal fin. Vertebrae 

 more or less calcified. 



Dorsal spines without lateral grooves; no notch below the terminal part of the 

 caudal 

 inner angles of pectorals not produced 



body subtriangular, a dermal fold at each side 



teeth unlike; first dorsal spine inclined forward 



Oxynotus (page 189) 

 body subround; dermal folds on the tail, if present 



teeth alike in the two jaws . . . Squalus (page 191) 



