The Crossopterygil 233 
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 pseudobranchie, but rather with the external gills 
of larval sharks. The vertebrae are very numerous and bi- 
Fie. 167.—FPolypterus congicus, a Crossopterygian fish from the Congo River. 
Young, with external 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 are 
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, 
Fia. 168.—Polypterus dethezi 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 cesophagus from below with a slit-like glottis. 
Unlike the lung of the Dipneusti, 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. 
