CLASSIFICATION OF DEVONIAN FISHES. 



39 



Tristichopterus, described by Sir Philip Egerton in the course of 

 the following Decade. 



GhGirolepis contains fishes with moderate-sized heads and 

 markedly heterocercal tails ; with a single dorsal fin, a single anal, 

 pectorals, and ventrals. The median fins are situated forwardly, 

 the dorsal being over the posterior part of the anal ; and the ventral 

 fins are so forward as to be almost close to the pectorals. None of 

 these fins are lobate. The body is covered with minute rhomboidal 

 scales, which do not overlap one another, so that the skin has 

 quite the aspect of shagreen. Nevertheless, according to Pander, 

 the structure of these bony scales is not so like that found in the 

 Squalidse as that of the scales of Diplacanthus. 



The head is usually crushed, and its component elements dis- 

 placed, but according to Professor Pander, whose account is largely 

 borne out by the specimens I have examined, the middle of the roof 

 of the cranium, from the posterior edge of the occiput to the an- 

 terior edge of the frontal region, is covered by two broad bony 

 plates, which were, perhaps, divided in the middle line. Pander 

 considers the anterior of these to be frontals, the posterior, to be 

 parietals. At the sides of the parietals lie three narrow bones, 

 which, perhaps, all belong to the skull, though the inner and upper- 

 most may appertain to the shoulder girdle. The anterior edges of the 

 other two bound the orbit posteriorly, and similarly elongated plates 

 lie in front of the eyes, beside the frontals. The upper jaw is a large 

 bone, rounded off posteriorly and tolerably broad behind, while 

 anteriorly its upper edge suddenly becomes excavated to form the 

 lower boundary of the orbit and then tapers ofi* ; it is beset with 

 small sharp conical teeth. The gape extends very far back, and the 

 lower jaw is a long flat bone toothed like the upper. 



According to Agassiz, there were larger teeth interspersed among 

 the smaller ones ("Recherches,'' p. 180 ; "Vieux Gres Rouge,'* p. 44), 

 but all in a single row. Like Pander (1. c, p. 73), I have been 

 unable to discover these larger teeth. The opercular apparatus and 

 the branchiostegal rays, or their representatives, were not observed 

 by Pander, nor have I seen indubitable evidence of their characters ; 

 but Agassiz (" Recherches,'' p. 132) has described and figured 

 the branchiostegal rays of Ch. Uragus. " The branchiostegal rays 

 " are very well preserved on both sides of the head ; the anterior are 



shorter and larger ; they are well seen on the left side. The 



posterior ones, which are better preserved on the right side, are 

 " narrower and more elongated. I count at least ten of them/' 



According to Pander a l^rge perforated plate surrounds the eye. 



