ORDERS OF FISHES. 157 



only living representatives of this group are the Port Jackson 

 Sharks {Cestracion), characterised by their pavement of plate- 

 like crushing teeth, adapted for comminuting small Molluscs 

 and Crustaceans. They are exclusively inhabitants of the 

 Australian and Chinese seas, and are remarkable for their 

 close resemblance to a large group of extinct forms, of which 

 the best known are the genera Hyhodus and Acrodus from 

 the Secondary rocks. 



The Gestraphori are known in a fossil state mainly by 

 their fin-spines, or " ichthyodorulites," and their teeth. It is 

 obvious, however, that it must be often very difficult or alto- 

 gether impossible to determine absolutely whether a spine or a 

 piece of shagreen belongs to a Cestraciont or to a true Shark. 

 Some of the forms, therefore, to be immediately mentioned, 

 must be regarded as being only provisionally placed amongst 

 the Cestraciontidce. 



With this proviso, the earliest known traces of Cestraciont 

 fishes appear to be in the Upper Ludlow rocks, at the sum- 

 mit of the Silurian series. Here, within the limits of a 

 single stratum, well known as the " bone-bed," occur remains 

 which have been with more or less probability referred to 

 Cestracionts. Some of these (fig. 520) are in the form of 



Fig. 520. — A, Spine of Oiiclms temiistriatiis ; b, Shagreen-scales of Theloclus. Both from 

 the bone-bed of the Upper Ludlow rocks. 



compressed, slightly curved spines, the sides of which are 

 grooved longitudinally. These have been referred to a 

 provisional genus, under the name of Onchus, and there 

 appears to be little doubt as to their truly belonging to 

 fishes of some kind. It is, however, quite possible that they 

 really belong to Pteraspis, in which case they must be re- 

 moved from their present place. 



Along with the spines of Onchus are found fragments of 

 prickly skin or shagreen, which have been referred to the 

 temporary genus Sphagodus, along with minute cushion- 

 shaped bodies, which are doubtless placoid scales, and which 



