Ichthyopsida 421 



have survived since the Mesozoir. Two existing African fishes, Polyp- 

 terus (Fig. 329D) and Calamoichthys, have been commonly re- 

 garded as being survivors of the group. Recent studies, however, favor 

 assigning these two fishes to the "ray-fins" — in spite of the fact that 

 the pectoral fin is prominently "lobed." Leaving Latimeria and the 

 two problematic African fishes out of account, all modern Osteichthyes 

 except lungfishes are certainly actinopterygians. The "lobe-fin" and 

 "ray-fin" fishes are commonly comprised together under the name 

 Teleostomi (meaning "terminal mouth"), referring to the fact that 

 in most of them — but it is not true of all — the mouth is at the tip of the 

 head, in contrast to the shark's mouth, which is usually far back on 

 the ventral surface of the head. The Osteichthyes are separated into 

 groups defined mainly on the basis of the degree of ossification of the 

 skeleton. 



The possession of functional lungs, together with other peculiar- 

 ities, including the three-chambered heart, is regarded as sufficient 

 grounds for putting lungfishes in a group by themselves. 



If ancient fishes be taken into account, fossils reveal the existence 

 of an abundant and highly diversified Paleozoic piscine fauna, the chief 

 point of resemblance of whose members was possession of a more or 

 less complete and usually heavy dermal armor consisting of either 

 small bony scales or large plates of bone. Because of this common 

 feature, the whole otherwise motley population is put under the name 

 Placodermi. Of these placoderms, some (the Acanthodii: Fig. 320B) 

 were externally suggestive of sharks, and others (Stegoselachii) were 

 dorsoventrally flattened and broadened in the manner of modern 

 skates (Fig. 330). Most of them were small (less than a foot long), 

 but among the Arthrodira were fishes which ranged up to a length of 

 perhaps 30 feet. 



Possession of a jaw skeleton of a somewhat simple and primitive 

 type sharply distinguishes the Placodermi from the agnathous Ostra- 

 codermi. Further, unlike the Agnatha, the placoderms usually had 

 well-developed pectoral and pelvic fins and in some cases (Acantho- 

 dii) several, up to five, pairs of relatively small fins between the pec- 

 torals and pelvics (Fig. 320B). The skeleton of the notochordal axis 

 seems to have been poorly developed. In some cases, evidence of bony 

 neural arches, and sometimes centra, is found. 



The placoderms were literally "bony fishes," but their boniness 

 was chiefly dermal. The pattern of their elaborate exoskeleton was so 

 unlike that of the dermal bones of modern fishes that it is most unlikely 

 that there is any direct phylogenetic relationship between the Placo- 

 dermi and modern Osteichthyes. The placoderms apparently comprised 



