84 FISH-EEMAINS OF THE CARBONIFEROUS ROCKS. 



Xystrochis, and Sandalodus, while some of the teeth named 

 Helodus are very probably derived from the anterior part of 

 the jaws of fishes of this famity. Among these the huge 

 teeth of Sandalodus are noteworthy. A tooth of this genns 

 in the cabinet of University College, Bristol, is triangular 

 in shape, the sides of the triangle being about twice as 

 long as the base. The crown is undulating, and towards the 

 broader end layers of growth are readily discernible. The 

 whole surface is pitted with small depressions. The length 

 of the tooth is 3 1 inches. It was probably the upper dental 

 plate of the fish. Commoner, if less striking, are the teeth 

 of Psephodus^ the margin of the crown of each tooth being 

 more or less crenulated. On the other hand, the teeth of 

 Tomodus^ also a not-uncommon form, are characterized by a 

 straight margin with an abrupt edge. The crown is smooth, 

 punctate, and gently arched with an outer apex strongly 

 inrolled. In Deltodus there are numerous rounded trans- 

 verse ridges and furrows running parallel to the inner border ; 

 while in Deltoptycliius there is one large broadening ridge, 

 extending from the outer apex to the inner margin, an 

 abrupt up-ridging of the borders of the tooth giving it a 

 characteristic twisted appearance. The type specimen in 

 the Bristol Museum, described by Agassiz as Coddiodus 

 contortus {Pscnnmodus contortus on the plate), is now re- 

 ferred to as Deltoptychius glbheridus. 



The family Cestraciontidce is represented to-day by the 

 before-mentioned Port Jackson shark of the Australian seas. 

 To this family belongs the genus Orodtts, whose large and 

 handsome teeth are often found in the quarries in the black- 

 rock limestone. The teeth are elongated from side to side, the 

 crown gently rising towards an eminence in the middle. Its 

 surface is characteristically marked with wrinkles and fur- 

 rows radiating towards each long margin. By the abrasion 



