1896. ] ANATOMY OF THE HOATZIN. 623 
Geniohyoid.—This muscle is in two distinct portions. The 
posterior division arises from the outer side of the ramus of the 
jaw, behind the anterior mylohyoid; it passes dorsally to both 
divisions of the posterior mylohyoid, and, running inwards and 
backwards, wraps round the ceratohyal to the tip. The anterior 
portion arises from the inner side of the ramus of the jaw, its edge 
being superficial to the mylohyoid anterior; it then runs forwards 
and inwards alongside the posterior division of this muscle, and is 
inserted to the ceratohyal, partly under and partly distally to the 
insertion of the posterior division. Gadow (‘Das Thierreich,’ p. 313) 
states that the geniohyoid is double in Nectarinia, Otis, Parrots, 
and Rhea; single in Corvus, Anser, Procellaria, and Spheniscus. 
Beddard and I found it single in Palamedea; I myself have found 
it single in Chauna, Rhytidoceros, Cygnus, Pelecanus, and Lopho- 
phorus ; double in Struthio, Dromeus, Rhea, Rhynchotus, and Ciconia. 
It appears as if this muscle were comparable with the latissimus 
dorsi; originally a diffused sheet it tends to break up into two 
discrete bands, but there are not sufficient data to draw any infer- 
ences of taxonomic value from its double condition in Opisthocomus. 
The researches of Garrod showed that, in the case of certain 
notable thigh-muscles, completeness of muscle formule was, on 
the whole, primitive, while incompleteness was secondary. In the 
attempt to extend this view to other muscles it is necessary to 
remember that many muscles are in process of splitting, and that 
in these cases increase in number is a sign, not of primitive, but of 
derivative character. 
Genioglossus.—At the most this is represented by a few fibres. 
Ceratoglossus.—This is a very strong muscle, arising from the 
outer side of the ceratohyal, anterior to the geniohyoids ; it passes 
forwards, superficially to the anterior division of the mylohyoid 
posterior, and deeply as regards the mylohyoid anterior ; ending in 
a round tendon, it is inserted along the side of the tongue almost 
to the tip. There is no trace of the division into two, which occurs 
in Fowls. 
Ceratohyoid.—This is a strong wide muscle running from the 
inner side of the ceratohyal, opposite the insertion of the foregoing 
muscle to the urobyal. 
The hypoglossals and the system of the sterno-hyod were 
present, but the individual muscles were not segmented from each 
other. 
Depressor mandibule.—A single very large muscle, of which the 
internal portion is more tendinous, runs from the lateral posterior 
and under surface of the occiput to the posterior and ventral part 
of the lower jaw. In Ducks and Geese this muscle is represented 
by three distinct portions, all of which Beddard and I found in 
Palamedea', and described as biventer and digastric. In the Fowls 
there are at least two portions separable; in Opisthocomus the 
tendinous inner portion no doubt represents an inner portion, 
1 “On the Anatomy of Palamedea cornuta,” P. Z.S. 1894, p. 536. 
