946 CLASS PISCES. 



viduals. Smaller teeth (fig. 873) also occur commonly in the 

 Miocene of Malta, the Tertiary of New Zealand, and still smaller 

 forms (fig. 874) in the Lower Miocene and Eocene of the Con- 

 tinent ; and the genus is also represented in the 

 Pliocene of Burma. Its earliest representative 

 is, however, C. longidens, from beds which appear 

 to be of the age of the Maastricht or topmost 

 Cretaceous. Small compressed and triangular 

 teeth, usually with serrated edges, from the Chalk 

 of Europe and India, to which the name Corax 

 cIrcVr^ T 7r^u/. ha * ^ en applied, probably indicate Sharks allied 

 tus, from the Lower to Carcharodon, although in external contour 



Miocene of the Con- . . °. _ _ . 



tinem. Reduced. they approximate to the teeth of the next 



family : they have, however, solid crowns. The 

 genus Cetorhinus (Selache), now represented by the huge Basking 

 Shark, dates from the Pliocene. 



Family Carchariid^:. — The last family we have to mention is 

 distinguished from the preceding by the presence of a nictating 

 membrane to the eye, and also by the hollow crowns of the teeth. 

 It is unknown before the Upper Cretaceous, and is dominant at the 

 present day. The teeth have triangular and compressed crowns, 

 usually with more or less distinctly serrated edges. The genus 

 HemipristiS) of the Upper Chalk and Lower Tertiary, is charac- 

 terised by its tall lanceolate teeth, the crowns of which have both 

 edges coarsely serrated, except at their summits. The existing 

 genus Galeocerdo is first recorded from the topmost Cretaceous of 

 Holland, and occurs throughout the European and American Ter- 

 tiaries from the Eocene upwards ; the existing forms being known as 

 "Topes." Carcharias, including the well-known Blue Shark, has 

 small and generally triangular teeth, those of the upper being very 

 different from those of the lower jaw. It may be divided into 

 several groups from the structure of the teeth, which in some forms 

 have smooth edges. It first occurs in the London Clay, and is 

 thence found throughout the European Tertiary series ; it has also 

 been recorded from the Egyptian Eocene, and is found in the fresh- 

 water Pliocene Siwaliks of north-western India. The strange Ham- 

 mer-headed Shark, the only representative of the genus Sphyrna 

 (Zygcena), has teeth so closely resembling those of Carcharias that 

 it is almost impossible to distinguish detached specimens. It ap- 

 pears, however, that there is sufficient evidence to prove the exist- 

 ence of a species of the former genus in the Miocene of Europe and 

 of the United States. 



Ichthyodorulites. — In conclusion, a brief notice may be given 

 of a few of the numerous genera founded upon the so-called " ich- 

 thyodorulites " or spines, of which the serial position cannot at 



