1UM DIPNOI. 147 



palato-quadrate arch on each side to near the articular surface 

 of the mandible. In the middle of the roof of the mouth, di- 

 vergent, cutting, dentary plates are developed upon it. An 

 osseous nodule lies in the articular head of the palato-quadrate 

 cartilage, and is continuous with the bone J^l 



The mandible presents dentary plates corresponding with 

 those of the palate, and biting between the latter. The hj'oi- 

 dean arch is attached to the posterior and lower edge of the 

 Buspensorium — which bears a bony ray representing an oper- 

 culum — while the hyoidean arch itself carries a single bran- 

 chiostegal ray {Jir, Fig. 53). 



The pectoral arch is composed of a median cartilaginous 

 part, with two lateral portions of cartilage, at once separated 

 from, and connected with, the median cartilage by bone. The 

 bone is separated from the cartilage by a layer of connective 

 tissue, and seems to represent the clavicle, while the cartilage 

 answers to the coalescent coraoo-scapular cartilages of other 

 fishes. 



The filiform fin is supported by a many-jointed cartilagi- 

 nous rod, articulated proximally with the coraco-scapular. 

 Upon this are disposed fine fin-rays like those of the Elasmo- 

 branchs, which support the marginal fringe of the fin. The 

 ventral fin has the same structure as the pectoral. 



The intestine possesses a spiral valve, and the rectum 

 opens into a cloaca. The lungs have remarkably stiff walls, 

 and extend through the greater part of the body, beneath the 

 spine. The glottis, opening upon the ventral wall of the 

 gullet, places them in communication with the cavity of the 

 mouth, into which the nasal sacs open by posterior apertures, 

 which lie inside the upper lip and constitute true posterior 

 nares. The heart has a small, but distinct, left auricle, into 

 which the blood which has been aerated in the lungs is re- 

 turned. In addition to lungs, Zepidosiren possesses both in- 

 ternal and external gills, but the latter are rudimentary in the 

 adult. 



The diflFerent species seem to differ in the manner in which 

 the primitive aortic arches are metamorphosed ; but it may be 

 said, generally, that the first has disappeared ; the second sup- 

 plies an internal branchia developed upon the hyoidean arch ; 

 the third gives off the anterior carotid artery, and supplies 

 neither internal nor external branchia ; the fourth supplies 

 only the first external branchia ; the fifth and sixth supply 

 both internal and external branchise ; while the seventh is 

 connected only with an internal branchia. The pulmonary 



