EASTMAN : STRUCTURE AND RELATIONS OF MYLOSTOMA. ] 7 



of Coccosteus are to be sought in the plates which Traqnair lias named 

 autero-lateral and antero-dorso-lateral. This identification we are pre- 

 pared to accept in part only. The antero-dorso-lateral, wo believe, 

 must be excluded from association with opercular elements, on account 

 of differences in form and relative position ; because it is articulated 

 with the headshield ; and also because its dorsal and ventral margins are 

 overlapped by contiguous plates. Besides, it is traversed by sensory 

 canals in a manner quite unusual for the operculum. 



Otherwise is the case with the antero-lateral, or " clavicular," as the 

 corresponding plate is called in Dinichthys. It has a somewhat similar 

 configuration, is of about the some relative size, and occupies the same 

 relative position both in these genera and in Neoceratodus. Its upper 

 portion overlaps the postero-lateral margin of the headshield for a 

 short distance behind the prominent postero-lateral angles, and its 

 lower front portion extends forward so as to continue the contour of the 

 lower jaw with scarcely appreciable interruption. Another important 

 fact to be observed is that this plate, both in Coccosteus and Dinichthys, 

 occurs in association with a small rod-shaped or spiniform piece, lying 

 immediately underneath, which is highly suggestive of the suboperculum 

 in typical Dipnoans. First observed in Brachydirus, where for lack of 

 a more appropriate name it was called " Euderorgan " by von Koenen, 

 this bone was afterwards detected in two species of Coccosteus by 

 Traquair, who named it "lateral spine," and compared it with the fixed 

 spinous appendage of Phlyctaenaspis and Acanthaspis.-^ We misdoubt 

 greatly whether the implied homology exists. Rather it seems to us 

 that the fixed spinous appendage of the two last-named genera should 

 be regarded as an elongated process of the ventral system of plates, 

 whereas in Coccosteus, Brachydirus, and Dinichthys the lateral spine 

 (= " pectoral fin-spine " of Newberry) is free, and meets all theoretical 

 requirements for a suboperculum. There is reason to believe, therefore, 

 that the antero-lateral and " lateral spine " of Coccosteans correspond, 

 respectively, to the operculum and suboperculum of typical Dipnoans ; 

 and the branchial aperture may be supposed to have been placed in the 

 prominent sinus formed by the lateral armoring of the trunk immedi- 

 ately behind these plates. This is also the region where we should 

 expect pectoral fins to have been attached, were such structures devel- 



1 Traquair, R. H. Notes on the Devonian fislies of Campbelltown, etc. Geol. 

 Mag., 1893, ser. 3, 10, p. 149. Notes on Palaeozoic fishes. Ann. Mag. Nat. Hist., 

 1894, ser. 6, 14, p. 370. Tiiese are apparently the references alluded to by Jaekel 

 in the postscript to his paper on Coccosteus, loc. cit., p. 116. 

 VOL. L. — No. 1 2 



