. Chordate morphology. Morphology (Animals); Chordata. small ganoid scales. The eye was large and far forward on the head. The operculum was large but separate septa ex- tended out from each of the hyoid and branchial arches. The eye was encircled by five plates; the operculum was supported only by branchiostegal rays. The most primitive ray-finned fishes are represented by such genera as Cheirolepis and Perleidus (Figure 2-16). These had strongly developed dermal bones covering the head, a rhomboid cover of thick scales with a distinct glassy enamel layer, a single dorsal fin, a heteroceral t
. Chordate morphology. Morphology (Animals); Chordata. small ganoid scales. The eye was large and far forward on the head. The operculum was large but separate septa ex- tended out from each of the hyoid and branchial arches. The eye was encircled by five plates; the operculum was supported only by branchiostegal rays. The most primitive ray-finned fishes are represented by such genera as Cheirolepis and Perleidus (Figure 2-16). These had strongly developed dermal bones covering the head, a rhomboid cover of thick scales with a distinct glassy enamel layer, a single dorsal fin, a heteroceral tail, an anal fin, and two pairs of lateral fins. The membranous part of the fins was supported by jointed fin rays. There was a small fleshy lobe at the base of the fin. Acenlrophorus (Figure 2-17) is the earliest known holostean fish, and in it the holostean features of cheek and jaw were fully developed. In the somewhat later occurring Walsonulus, or Parasemionotus, there was an interesting combination of features of the holostean and palaeoniscoid. The crossopterygian fishes (Figure 2-18) are choanate fishes. They are best represented by Osteolepis and Eusthenopteron, which show many similarities to the assumed ancestor of the Amphibia. These fishes had thick dermal bones cover- ing the head and operculum. The body was covered by rhomboid or cycloid scales of the cosmoid type (based on their histology—see Chapter 8). Each had two dorsal fins, a heterocercal or diphycercal (double) tail, an anal fin, and two pairs of lateral fins with fleshy lobes. From these fleshy lobes with their "fringe" of web the term Crossopterygii is derived. In these fishes the fleshy lobes are not nearly as large as those of the dipnoans, nor more extensive than those of Latimena. Dipterus was an early Devonian member of the Dipnoi. The body was covered by cycloid scales of the cosmoid type. There was the same pattern of fins as that seen in the crossopterygian fishes. The earliest of th
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