488 PROCEEDINGS OP THE ACADEMY OF 



Triakis semifasciatus Girard, explorations and Surveys for a Railroad Route, &c. 

 vol. x. Fishes, p. 3G2. 



Family HETERODONTOID^J Gill. 



, * " t Cuvier, Regne Animal, tome ii. 1817. 



Squalus ) 



Cestraciontes Agassiz, Poissons Fossiles, tome ii. 1833. 



Squalidae verse (Cestracionini) Bonaparte, Selachiorum Tabula Analytica. p. 5, 



1838. 

 Squalidae (Centrininae) Swain-ton, Natural History of Fishes, &c, vol. ii. p. 



1839. 

 Cestraciontes Mailer and Henle, Systematische Beschreiburg der Plagiostomen, 



p. 76, 1841. 

 Cestraciones Miiller, Arc. 1, 1317, 1845. 

 Cestraciontidae Owen, Lectures on the Comparative Anatomy and Physiology of 



the Vertebrate Animals, p. 51, 1846. 

 Squalidae (Heterodontina) Gray, List of the Specimens of Fish in British Mu- 

 seum. Chondropterygii, p. , 1857. 

 Cestraciontoidaa Bleeker, Systematis Piscium Naturalis Tentamen. 

 Heterodontoidae Gill, Analytical Synopsis of the Order of Squali. p. 29, 30, 37, 



1862. 

 Squalidaa (Cestraciontini) Bonaparte, Syst. Vert. 



Body elongated and obtusely trihedral, gradually tapering from the anal re- 

 gion towards the caudal fin. 



Scales very small. 



Head high, with the forehead declivous and the snout little prominent. 



Eyes lateral, but very high on the sides; nictitating membrane obsolete. 



Mouth subterminal but inferior and more or less arched in front. 



Teeth in front compressed and trenchant or digitated, on the sides arranged 

 in whorls, paved and adapted for grinding. 



Nostrils continued backwards to the mouth. 



Spiracles small. 



Branchial apertures five, moderate or small; the last above the base of the 

 pectoral fin. 



Dorsal fins two, each well developed and with a spine enveloped in the front 

 of its margin; the anterior angle of each is rounded, and the posterior acute; 

 the first fin above the interval betweeu the pectoral and ventral fins; the second 

 more or less behind the ventral fins, and remote from the caudal. 



Anal fin small or moderate, below or behind the second dorsal fin, and re- 

 mote from the caudal ; the anterior angle is rounded but produced, and the pos- 

 terior blunt. 



Caudal fin heterocercal ; the upper lobe moderate and with its under edge 

 notched and lobed nearer the end, and with the portion above the ventral 

 column enlarged ; the lower lobe is small or moderate. 



Pectoral fins normally developed, with each angle rounded, but towards the 

 anterior produced. 



Ventral fins moderate, inserted nearer the head than the tail, with each angle 

 obtuse. 



The characters of the family of Heterodontoids as here exposed are derived 

 almost entirely from our knowledge of the species living at the present day. 

 The earliest known living representative of the family, the Port Jackson shark, 

 has become celebrated on account of the views of Agassiz, by whom it was con- 

 sidered as the type and sole existing representative of a family rich in peculiar 

 genera and species at former epochs of the world's history. That naturalist 

 has proposed to refer to the family of Cestraciontes, numerous vestiges of the 

 representatives of the order of Squali, found in every formation from the earli- 



[Oct. 



