ORDERS OF FISHES. 153 



tubercles, or spines, composed of bony or dentinal matter, 

 and scattered here and there in the integument. In the 

 case of the Eays, these placoid ossifications often take a very 

 singular shape, consisting of an osseous or cartilaginous disc, 

 from the upper surface of which springs a sharp recurved 

 spine, composed of dentine. The so-called " shagreen " of 

 the Dog-fishes and Sharks is composed of very small and 

 close-set tooth-like processes. At other times the placoid 

 structures are developed into "dermal defences" or "ichthyo- 

 dorulites." The minute structure of these exoskeletal struc- 

 tures is closely or entirely similar to that of the teeth. In 

 some cases the exoskeleton is absent. 



2. The gills are fixed and pouch-like, and differ very 

 materially from those of the Bony and Ganoid fishes. In 

 the case of the Sharks and Eays, the gill-pouches open upon 

 the surface by a series of separate apertures, which are placed 

 on the sides of the neck in the former, and on the under 

 surface of the body in the latter. In neither is there any 

 gill-cover or operculum, nor are there any branchiostegal 

 rays. In one section of the order, however — viz., the Holo- 

 cephali — though the internal structure of the gills is essen- 

 tially the same as in the Sharks, there is only a single 

 branchial aperture or gill-slit externally, and this is pro- 

 tected by a rudimentary operculum and branchiostegal rays. 



The order Masmohranchii is divided into the two sub- 

 orders of the HolocejjJiali and Plagiostomi. The former 

 comprises the living Chimcerm, characterised by the mouth 

 being terminal, and by there being only a single gill-sht. 

 The latter comprises the living Port Jackson Shark, the true 

 Sharks and Dog-fishes, and the Eays, characterised by hav- 

 ing the mouth transverse and placed on the under surface of 

 the head, whilst there are several apertures to the gills. 



As regards their general distribution in time, the Elasmo- 

 hranehii are nearly as ancient as the Ganoids. At the top 

 of the Upper Ludlow rocks, or at the close of the Upper 

 Silurian epoch, there have been discovered the remains of 

 undoubted Plagiostomous fishes, most nearly allied to the 

 existing Port Jackson Shark {Ccstracion FMlijjpi). These 

 remains consist chiefly of defensive spines, which formed the 



