On the Cestracion Philippi {Port Jackson Shark). 15 



images, and the appearance of water. On advancing up the 

 ascent, so as to raise the eye, and thus diminish the angle of 

 incidence of the rays which entered it, the lake contracted, 

 appeared discontinuous, as if studded with islands, and dis- 

 appeared. It reappeared again on placing the eye in the first 

 position. 



The same appearance was observed on the following Sun- 

 day, under similar circumstances. 



The phenomenon is a well-known one ; but every instance 

 of its ocurrence in a locality where it has not been previously 

 observed is worthy of record, and the collection ' of trivial 

 scientific facts is one of the objects of a scientific society. 



Art. IV. — On the Cestracion Philippi (Port Jackson Shark), 

 Trigonia and Terebratula of the Australian Seas. By SiZAR 

 Elliott, Esq. 



[Read before the Institute 4th March, 1857.] 



This fish belongs to the Placoid order, or Sharks, (by the 

 arrangement of M. Agassiz), and forms the only living example 

 of the family of Cestracions (Cestraciontid<e), the members 

 of which are distinguished from all other sharks by having the 

 teeth flattened,' and formed for grinding. Numerous genera of 

 this family are abundant in the Mesozoic rocks of Europe, and 

 all Naturalists and Geologists seemed hitherto to have been of 

 opinion that they abound also in the Palaeozoic Rocks ; but 

 Professor M'Coy, in his last work on the " British Palaeozoic 

 Fossils," proves that the succession of the teeth in the 

 Mountain Limestone Genus Cochliodus (which was hitherto 

 supposed to be a fossil-shark's jaw of the type of cestracion, 

 and which furnished the key to Agassiz for grouping all the 

 Palaeozoic blunt fish-teeth of the Palaeozoic Rocks with the 

 Cestracions), was from below upwards, and that they were 

 not on a rotating membrane, succeeding each other from 

 behind forwards, so that, according to Professor M'Coy, the 

 supposed Palaeozoic cestracions belong in fact to the bony 

 fishes, and not to the sharks at all: in other words M. Agassiz 

 is in error in grouping the blunt fish-teeth of the mountain 

 limestone with the blunt fish-teeth of the Mesozoic Rocks ; 

 the former having their teeth progressing from behind for- 

 wards, the latter from below upwards. 



However the fact of greatest interest remains of the great 



