G YMNOPHION A . 301 



arranged in transverse rows. The eyes are small, functionless 

 and covered by the skin ; though reduced in size, all the usual 

 parts are present. There is on each side of the head behind the 

 external narial opening a small pit which penetrates the maxillary 

 bone and contains a protrusible tentacle-like structure (Fig. 173, 

 T). A well developed harderian gland pours its secretion into 

 this pit. There is no tympanum or tympanic cavity. The 

 anus is close to the hind end of the body, the caudal region being 

 very small or absent. The notochord is persistent (except in 

 the centre of the vertebrae), and the vertebrae are biconcave and 

 numerous, mostly with ribs, which do not meet to form a sternum. 

 The skull (Fig. 173) is compact and has a complete bony roof. 

 The parietals and frontals are separated and there is sometimes 

 a small bone called the turbinal or lateral nasal just external 

 to the outer nostrils (shown, but not marked in Fig. 173). The 

 jugal arch is well developed and joins the parietal and frontal, 

 thus roofing over the temporal fossa ; it reaches back to the 



FlQ. 172. Siphonops mexicana (Regne animal) from Claus. 



suspensorium, which appears to consist of fused squamosal and 

 quadrate. The vomer is double and bears teeth. The palatine 

 also bears teeth and extends back on the inner side of the maxilla 

 with which it is continuous. There is a prefrontal, and some- 

 times a postorbital (postfrontal) which nearly encircles the 

 orbit. The stapes is perforated and is connected with the 

 suspensorium (quadrate). The orbit is small, and the maxilla 

 is perforated by a pit for the tentacular organ. The mandible 

 in some forms has two rows of teeth. The remains of the hyoid 

 and three branchial arches are found in the adult, the hyoid 

 and 1st branchial being connected to the same median piece. 

 The lungs are asymmetrical as in the snakes, the right being 

 much larger than the left which is more or less atrophied. 



The conus arteriosus possesses two rows (Ichthyophis) or one 

 row (Siphonops} of valves, and is without a spiral valve. The 

 ventral aorta is long. There are only two pairs of aortic arches. 

 Of these the first is the systemic (2nd branchial) which gives off 

 the carotid, and the second is the pulmonary. They are joined 



