Gill Arches of Teleostean Fishes — Nelson 399 
Figs. 26-30. 26 and 27, Synaphobranchus affinis. 28, Simenchelys parasiticus. 29 and 30. Dysomma an- 
guillare. 
in which the hypobranchials are characteristic- 
ally anteriorly directed. The gill skeleton of 
Simenchelys is the most generalized in terms of 
the number of elements (Table 1), yet that of 
Synaphobranchus has the lower tooth plates in 
four pairs. Other such multiple tooth plates 
occur in such lower teleosts as Osteoglossum, 
Hiodon, Elops, and Albula, but not generally 
in other teleosts nor in the other eels examined. 
In Conger, however, the lower tooth plates are 
initially in two pairs which later fuse together 
and with C5 during ontogeny to form a single 
pair (personal observations). The multiple tooth 
plates of Synaphobranchus, therefore, seem to 
be primitive features and are evidence against 
the derivation of Synaphobranchus from such 
a form as Conger. For this reason it seems 
appropriate to consider the synaphobranchoid 
lineage as possibly equivalent to the other two, 
the congroid and the anguilloid. 
Specimens of llyophis have not been available 
for study. The genus is included in this lineage 
primarily on the evidence of Castle (1964), 
who included llyophis in the family Synapho- 
branchidae. 
Anguilloid Lineage 
The arches of Heterenchelys and Anguilla 
(Figs. 31, 32) are quite similar and are the 
most generalized. All of the skeletal elements, 
found in eels are present in Heterenchelys, 
while Anguilla lacks only B3 (Table 1). Ac- 
cording to Norman (1926) even this element 
occurs in cartilaginous form in the embryo. 
The arches of Heterenchelys are much nearer 
those of Anguilla than those of Morin gua 
(Figs 33, 34), which has them noticeably 
reduced, basibranchials being either rudimentary 
or absent. Dorsally, 1 2 has lost its usual con- 
nection with the proximal end of El. Gill arch 
characters, therefore, suggest that Heterenchelys 
is more closely related to Anguilla than to 
Morin gua (cf, Regan, 1912^:32). 
The xenocongrids, Dysommina, and the 
muraenids are alike in having lost the entire 
basibranchial series. While the hypobranchials 
of either side retain midventral connections in 
Dysommina (Fig. 36), they are without such 
connections in the xenocongrids (Figs. 37-40) 
and muraenids (Figs. 41, 42). Among xenocon- 
grids and Dysommina, C5 is present and ossi- 
