The Crossopterygii 



607 



It is inserted, not on the gill-arches, but on the hyoid arch. 

 Its origin is from the external skin. It can therefore not be 

 compared morphologically with the gills of other fishes, nor 

 with the pseudobranchitE, but rather with the external gills 

 of larval sharks. The vertebrae are very numerous and bi- 



FiG. 378. — Polypierus congicus, a Crossopterygian fish from the Congo River. 

 Young, with e.\ternal gills. (After Boulenger.) 



concave as in ordinary fishes. Each of the peculiar dorsal 

 spines is primitively a single spine, not a finlet of several pieces 

 as some have suggested. The enameled, rhomboid scales ari 

 in movable oblique whorls, each scale interlocked with its 

 neighbors. 



The shoulder-girdle, suspended from the cranium by post- 

 temporal and supraclavicle, is covered by bony plates. To the 

 small hypercoracoid and hypocoracoid the pectoral fin is at- 

 tached. Its basal bones may be compared to those of the 

 sharks, mesopterygium, propterygium, and metapterygium, 

 which may with less certainty be again called humerus, radius, 



Fig. 'il%.—Polypteriu delhezi Boulenger. Congo River. 



and ulna. These are covered by flesh and by small imbricated 

 scales. The air-bladder resembles the lungs of terrestrial 

 vertebrates. It consists of two cylindrical sacs, that on the 

 right the longer, then uniting in front to form a short tube, 

 which enters the oesophagus from below with a slit-hke glottis. 

 Unlike the lung of the Dtpneusti, this air-bladder is not cellu- 

 lar, and it receives only arterial blood. Its function is to assist 

 the respiration by gills without replacing it. 



