A44 ELT S: [VoL. XIV. 
oder an den Unterkiefer’’ (No. 16, p. 46), and that in Amia the 
lower end of the preoperculum supports intimately the lower 
end of the symplectic, where that bone articulates with the 
mandible. If the lower portion of the dermal preoperculum of 
Amia should here fuse with the underlying cartilage bone, or 
simply replace it, both of which processes are said to be of 
frequent occurrence in other of the dermal bones, the quadrato- 
jugal of the Stegocephali would arise. This possible origin of 
the bone finds some support in Gaupp’s (No. 17, p. 104) serial 
grouping of the three bones called by him “Squamosum,” 
«Paraquadratum,” and “Quadrato-maxillare,” the Squamosum 
being the upper and the Quadrato-maxillare the lower element 
of the series. The fact that the preoperculum of fishes is 
formed by the fusion of several components, more or less nu- 
merous according to the number of sense organs that develop 
in the line, and the further fact that the organs of the line in 
Amia first appear at or near the middle point of the line, and 
develop or differentiate from there in both directions, may 
offer some explanation of the apparent disappearance, to which 
Gaupp calls attention, in certain animals or classes of animals, 
of one or the other of the three elements of this series. 
The quadrato-jugal of the Stegocephali is considered by Baur 
(No. 5) as the probable homologue of the suboperculum of fishes. 
The jugal of reptiles varies greatly in its relations to the 
adjacent bones. In Dimetrodon (No. 6) and in the crocodile 
(No. 17, Fig. 5) it lies wholly posterior to the maxillary bone, as 
it does in Trematosaurus. In Branchiosaurus (No. 16, p. 68, 
Fig. 30) it has nearly the same length as the maxillary and 
quadrato-jugal together, and lies immediately superior to them 
and parallel to them. In Chelydosaurus (No. 16, Bd. II, p. 21) 
it is separated from the lower edge of the orbit by an anterior 
prolongation of the postorbital bone, and is completely fused 
with the lachrymal. In this animal there are thus three parallel 
lines of bones below the orbit: the postorbital, the jugal and 
supratemporal, and the maxillary and quadrato-jugal. 
If the jugal bone of reptiles and the maxillary bone of 
Polypterus are, as here suggested, developed in part, in connec- 
tion with homologues of the horizontal cheek line and the sub- 
