Prof. Owen's Description of the Lepidosiren annectens. 347 



arches which support the gills, is subject to a previous filtration by the interlock- 

 ing- marginal denticles ; while that which flows between those two branchial 

 arches from which no gills are developed has a free and uninterrupted passage. 



The gills do not form any external projection, as in the gill-bearing Peren- 

 nibranchians, but are contained in a moderately capacious branchial chamber, 

 the parietes of which are formed by a mucous and muscular stratum*; the 

 external outlet is the vertical slit already described, situated immediately ante- 

 rior to the filamentary pectoral member. 



Thus although the organs for respiration through the medium of water corre- 

 spond in all essential points with those of the true Fishes, yet the gills approxi- 

 mate in their filamentary form to those of the Perennibranchiate Reptiles. And, 

 again, although the gills are four in number on each side, as in the Osseous 

 Fishes, yet the number of branchial apertures and arches corresponds with that 

 M'^hich characterizes the higher Cartilaginous Fishes. So that while we perceive, 

 even in the organs for breathing water, a tendency towards the amphibious 

 type, we find at the same time that the branchial as well as the osseous 

 system manifests a most interesting and hitherto unexampled transitional 

 structure between the Plagiostomous and Osseous Fishes. 



We have next to consider that part of the Respiratory System which is or- 

 ganized for breathing immediately the atmospheric air, or the Lungsf : for I 

 know not how otherwise to designate, according either to their physiological or 

 morphological relations, those organs, which in the technical language of the 

 ichthyologist would be termed the swim- or air-bladder. 



The tracheal, or, to use the same technical and partial nomenclature, the 

 ' ductus pneumaticus,' is a short wide membranous tube, as in the Perenni- 

 branchiate Reptiles. The glottis § opens near the posterior part of a long 

 rudimental thyroid cartilage ; a few lines posterior to the isthmus faucium 

 the opposite end of the trachea dilates into a membranous sac which com- 

 municates by two large lateral apertures with the lungs. These are widest 

 at their anterior extremities, and gradually decrease in diameter to the cloaca, 

 behind which they terminate each in an obtuse point. They are lodged in 

 the dorsal angle of the abdominal cavity behind the kidneys, and are attached 



* Tab. XXVI. fig. 2, h. f Tab. XXV. fig. 3, i, i. J Tab. XXVI. fig. 1, k. 



§ lb. fig. 1, e. 



