32 CONTRIBUTIONS TO NORTH AMERICAN ICHTHYOLOGY- -IV. 



30.— MICRISTODUS Gill, 18G5. 

 CGill, Proc. Ac. Nat. Sci. Pliila. 1865, 177: type MicrhtodMn pxuciatus Gill.) 



This geiins is known from its teeth only. These are described as fol- 

 lows: "The teeth are fixed and extremely minute, the largest little 

 more than a line in length (in a shark 20 feet long) and decrease 

 towards the ends of the jaw ; they are disposed in regularly transverse 

 rows, of which there are 104 to 167 on each side, while in front there 

 are 13 to 10 in each transverse row ; each tooth is recurved backwards 

 and acutely i)ointed, swollen and with a heel-like projection in front 

 rising'from its base." — (Gill.) [luy.pur^ small; ia-u-^ an uimght projection; 

 o'oot;-:, tooth.) 



40. M. piiBictatus Gill. 



A very large spotted shark found in the Gulf of Califoruia. ((xill, 

 Proc. Ac. Nat. Sci. Phila. 1805, 177.) 



Family XIV.— HETERODONTID^. 



{The Port Jackson SharJcs.) 

 Sharks with two dorsal fins, both provided with spines ; the first dor- 

 sal opi>osite to the space between pectorals and ventrals; the second in 

 advance of the anal. Body elongated, obtusely trihedral, gradually taper- 

 ing from the anal region towards the caudal fin; head high, with the fore- 

 head declivous, little prominent; nostrils confluent with the mouth; 

 mouth rather narrow, the upper lij) divided into seven lobes, the lower 

 with a fold ; spiracles small, below the lower part of the eye ; no nictitat- 

 ing membrane; gill-openings rather narrow; dentition simihir in both 

 jaws, viz, small obtuse teeth in front, which in the young are ]>ointed, 

 and provided with three to five cusps ; the lateral teeth large, pad-like, 

 twice as broad as long, arranged in oblique series, one series being formed 

 by much larger teeth than those in the other series ; caudal fin moderate. 

 Genera 1, 2, or 3 ; species about 4 ; inhabiting the Pacific Ocean. Of much 

 interest to i^aloeontologists from their supposed relation to certain extinct 

 types. {Cestraeiontidw Giinther, viii, 417, 418; but Heterodontus has 

 priority over Cestracion.) 



* Branchial region longer tliau high, the slits being little elongated ; molar teeth flat 

 and closely contiguons Gyropleurodus, 31 . 



31.— GYKOPI.EURODUS Gill, 1862. 

 ((iill, Proc. Ac. Nat. Sci. Phila. 1882, 489; type Centrdcion franckci Gixavd.) 



The characters distinguishing this group from Heterodontus are thus 

 given by Professor Gill : 



