FOSSIL FISHES. 



15 



Janassa (Fig. 17), etc. As their jaws were only cartilaginous, they 

 perished and the teeth are usually met with singly, so that their 

 correlation with each other and with the spines is always difficult. 



Nevertheless, the spines of Pleuraeanthus and the teeth oiDiplodus 

 (Wall-case 3 b, and Table-case 32), from the Coal-measures, are now 

 found to belong to one fish ; the spines of Ctenacantlius (Wall-case 1, 

 and Table-case 25), and teeth named Cladodus (Table-case 27), from 

 the Carboniferous Limestone, are also now correlated by some 

 authors. 



In Wall-case 2, and Table-cases 27, 28, and 30, is arranged a fine 

 series of remains of heads with the teeth, spines, and shagreen, of 

 Hybodus (Fig. 11) and Acrodus (Fig. 18), 

 the sharks of the Lias formation.* 



In addition to the dorsal fin-spines, of 

 which these genera possessed two, like the 

 recent Port-Jackson Shark, they were also 

 provided with four curious recurved booklets 

 (called by Agassiz Sphenonclius (See Table- 

 case 28). They have a large bony base which 

 was imbedded just above the eye on each 

 side of the head, and a second pair a little 

 farther back. 



The entire series of teeth in both Hybodus 

 and Acrodus is known (See Wall-case 2, and 

 Table-cases 28 and 30) ; they present a most 

 remarkable variation in form, between 

 sharply-pointed cuspidate teeth in front, 

 and flatter grinding teeth at the sides of 

 the jaws (Fig 18). 



We fortunately know also the nearly per- 

 fect jaw of StropJwdus (See Table-case 31), 

 from the Oolite of Caen, Normandy, showing 

 about sixty teeth in their true relation to 

 each other, and exhibiting the same great 



variation in form, which, if found separately, might have led to their 

 being referred to distinct species. 



It is interesting to notice also the variation in the number of 

 teeth in the jaws of these ancient Cestracionts. 



Thus Plethodus (Table-case 31), from the Chalk of Sussex, 

 appears to have possessed only one tooth in each jaw ; Ptychodus, 

 also from the Chalk (Table-case 31), has been found with as many 

 as 200 associated teeth. Streblodits and Coc7iliodus, from the Car- 

 boniferous Limestone, had only two or three teeth on each sidef 

 (Table-case 29). 



* These forms range upwards in diminished numbers to the Chalk formation. 



T But many of the palatal teeth of the Carboniferous Limestone which bear 

 separate generic names, may, when we know more of their true nature, be shown to 

 belong to one and the same fish, as is seen in the case of S'.rophodus, Acrodus, and 

 Hybodus, and in that of the modern Cestracion. 



Fig.IS.— Teeth of Acrodus, 

 Lias, Lyme Regis. Five 

 palatal teeth, showing vari- 

 ation in form. 



