82 



Elasmobranchii. 



and close set, as in the Dog-fish, this dermal covering is called 

 " shagreen." 



Those of the Elasmobranchs with lateral gill-clefts are 

 commonly known as " Sharks " (Fig. 109), while those with 

 depressed body and ventrally placed gill-clefts fall under the 

 denomination of "Rays" (Fig. 110). There are many inter- 

 mediate forms, however, which it is impossible to distinguish in 

 a fossil state. 



FIG. 109. Port Jackson Shark, Cestracion Philippi, Lacep., from Australia. 

 sp', anterior dorsal spine ; sp", posterior dorsal spine. 



From the perishable nature of their skeletal parts, it is obvious 

 that the paleeontological history of these fishes is most difficult 

 to decipher. In the majority of instances, the fossils consist 

 merely of detached spines, shagreen-granules, teeth, or pieces of 

 cartilage ; and it is often impossible to correlate these unsat- 

 isfactory fragments, so that the different parts of the same 

 species not unfrequently receive even distinct generic names. 



FIG. 110. The Bay (Raja Murrayi, Giinther), from Kergeulen's Island. 

 A, dorsal aspect; B, part of ventral side ; sp., spiracle; br., gill clefts; m., mouth. 



Sometimes, however, complete fishes are met with, and many 

 beautiful examples are shown from the Lias of Lyme Regis, the 

 Lithographic Stone of Bavaria, and the Upper Cretaceous of 

 Mount Lebanon. 



