HI PPOC AMPUS. DIODO N. 117 



an d in its thickness we find only vestiges of branchiostegous 

 rays. These fishes are also recognised by the mail-like plates 

 which cover the body, and render it almost always angular. 

 They are generally small. 



28. To this order belongs the 

 Hippocampus, (Fig. 59.) the body 

 of which is laterally compressed, 

 and more elevated than the tail ; 



on curling up, after death, the HIPVOCAMPU8 . 



head and trunk bear some resem- 

 blance to the neck of a horse in miniature, which has obtained 

 for this little fish the common name of Sea-horse. 



ORDER OF PLECTOGNATHI, 



29. The fishes composing this group form the connecting link 

 between the ordinary and cartilaginous fishes, as much by the 

 conformation of their jaws, as by the tardy consolidation of their 

 skeleton. Their chief distinctive character is, that the maxillary, 

 is solidly fi^ed upon the side of the intermaxillary bone, which 

 alone forms the jaw, and that the palatine arch articulates with 

 the cranium, in a manner which renders it immoveable. More- 

 over, their opercula and branchiostegous rays are concealed be- 

 neath a thick skin, which leaves externally only a small branchial 

 slit ; they have no true ventral fins ; and they have only vestiges 

 of ribs. This order comprises two families, recognisable by t'.ie 

 opening of their mouth, namely : the Gymnodontes, and the 

 Sclerodermi. 



30. In the FAMILY OP GYMNODONTES, there are no apparent 

 teeth ; but the jaws are furnished with a species of ivory beak, 

 internally divided into plates, which represent the teeth It 

 includes the Dindon, the Tetradon, the Mole,&c. 



31. The DIODON has received this name, because the jaws, 

 being undivided, have each but a single piece, while in the 

 TETRADON, tHfcy are divided in the middle by a suture, so as to 

 present the appearance of four teeth : two above, and two below. 

 These two genera of fishes have the faculty of swelling them- 

 selves up like a balloon, by swallowing air, -and thus distending 

 a first, very extensible stomach, which occupies the whole length 

 of the abdomen. This peculiarity has obtained for them the 



28 What is the Hippocampus ? 



29. What are the characters of the order of Plectognathi ? 



30. How are Gymnodontes characterised ? 



31. How does the Diodon differ from the Tetradon? What are the 

 peculiarities of these two genera ? 



