96 IOWA GEOLOGICAL SURVEY 



to the skull is usually hyostylic, in that the hyoinandibular and 

 palato-quadrate form a freely movable support for the mandible. 

 Only one or two genera form exceptions to this rule, where the 

 condition is that which Huxley has termed amphistylic (see ante, 

 p. 66), or as in the Port Jackson shark, there is a tendency 

 toward that complete fusion of the palato-quadrate cartilage with 

 the cranium which is known as autostyly, — when the mandible 

 articulates directly with the cranium.* There are from five to 

 seven branchial arches and clefts, and the latter are separated 

 by complete interbranchial septa. Arches are present for the 

 support of the paired fins ; the pectoral pair is with rare excep- 

 tions uniserial, and the pelvic pair invariably so. The exoskel- 

 etal supports of all the fins consist of numerous slender horny 

 fibres (ceratotrichia) and, when present, the fin-spines are in- 

 vested by enamel. Gaspers are generally present in the male. 



Besides the foregoing definition, it may not be uninstructive 

 to introduce at this point a few topics of general interest relating 

 to modern sharks and rays, which are extracted in slightly 

 abridged form from Professor Bridge's discussion in the Cam- 

 bridge Natural History volume on Fishes (1904, p. 432). 



The Elasmobranchs are for the most part predaceous fishes, 

 living at different depths in the sea, from the surface to nearly 

 a thousand fathoms, and ranging from mid-ocean to the shal- 

 lower waters round the coasts in almost every part of the world. 

 Although typically marine, they sometimes ascend rivers be- 

 yond the reach of tides, and a few are permanent inhabitants of 

 fresh water. They are most abundant in tropical and sub- 

 tropical areas, where they also attain their greatest size, and are 

 numerous in temperate regions, but there are some species which 

 are typically Arctic. None of them are small, and some of the 

 sharks are the largest of living fishes. All are carniverous, but 

 so diversified is their food that in different species it may range 



* Polio and some others are of the opinion that autostyly, whether incipient as 

 in Cestracion, or complete as in Chimaeroids and Dipnoans, is a secondary modi- 

 fication, which may be independently acquired in widely different groups of fishes, 

 and is usually associated with the need of a firm and rigid support for an except- 

 ionally massive dentition. Dollo's remarks on this subject are given as follows: 



"Sans vouloir absolument meconnaitre l'importance taxonomique de l'autosty- 

 lie, — je ne puis la considerer comme un caractere fondamental. C'est une pure 

 consequence de l'adaptation a un regime triturateur tres accentue (mylodonte), — 

 dans un but de consolidation de l'appareil masticatoire." — Sur In pltylogenie des 

 Dipneustes, Bull. Soc. Beige Geol. etc. 1895, 9. p. 109. 



See also a more recent communication by the same author entitled "Sur quelques 

 points d'ethologie paleontologique relatifs aux poissons." Ibid., 1906, 20, p. 1. 



