16 bulletin: museum of compakative zoology. 



act of sniffing ; and this function can only be performed effectually 

 when posterior nasal apertures are present. 



There remains finally the somewhat difficult task of homologizing the 

 opercular elements in Arthrodires and Dipnoans: difficult, because 

 more than one interpretation is open to us, and we cannot be entirely 

 certain as to which plate or plates of the Coccostean skeleton corre- 

 sponds to the two opercular bones found in typical Dipnoans. It is a 

 well-known fact that the suborbital in Coccosteus is succeeded behind 

 by a small, deep, semi-elliptical plate with free hinder margin, although 

 no such bone has been found in Diuichthys, and in Neoceratodus the 

 corresponding space is filled by cartilage or fibrous tissue. The bran- 

 chial aperture does not occur in this vicinity in the modern form, but 

 is placed considei'ably further back. The doubtful element referred to 

 in Coccosteus is lettered " x " in Woodward's restoration of that genus, 

 "J " in Traquair's, and in Jaekel's it is unmarked. The first-named 

 author suggests that it is " not improbably to be regarded as the opercu- 

 lum;"^ Traquair interprets it as jugal;^ Jaekel as quadrato-jugal.^ 

 Against its interpretation as an operculum it may be argued that the 

 bone in question is of disproportionally small size; is situated rela- 

 tively too far forwards, where we should expect the side wall of the 

 head to be closed ; is unaccompanied by any subjacent element answering 

 to the subopei'culum ; and has apparently no equivalent amongst other 

 Arthrodires. In Dinichthys, the corresponding space is covered by the 

 posterior portion of the suborbital, which extends as far as the postero- 

 lateral angles of the headshield, and is in close proximity below with 

 the so-called " clavicular." Judging from this fact, and from the con- 

 ditions observed in Neoceratodus, it seems preferable to regard the 

 bone " X " in Coccosteus merely as an intercalary piece which may 

 exist occasionally as a separate ossification, and serves to protect the 

 side of the head. The consequence of this view is that we shall be 

 compelled to search, as Jaekel has done, for the operculum and sub- 

 operculum amongst plates forming part of the lateral armoring of the 

 trunk. 



According to Jaekel's idea (Zoc. cii., p. 109), the opercular elements 



1 Woodward. A. S. Catalogue fossil fishes British Museum, 1891, pt. 2, p. 280, 

 Fig. 44. 



2 Traquair, R. H. On the structure of Coccosteus decipiens Agassiz. Ann. 

 Mag. Nat. Hist., 1890, ser. 6, 5, p. 127, Plate 10, Figs. 1, 2. 



3 Jaekel, 0. Ueber Coccosteus und die Beurtheilung der Placodermen. 

 Sitz. Gesell. Nat. Freunde, 1902, p. 108 (restoration, p. 107). 



