1896.] ANATOMX or the hoatzin. 623 



Geniohyoid. — This muscle is in two distinct poi-tions. 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 sup(>rficial 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 llhea ; single in Corvus, Anse.r, Frocellaria, and Sjiheniscu-i. 

 JJeddard and I found it single in Palame.dea ; I myself have found 

 it single in Ohaumi, Ehytidoceros, Cygnus, Pelecanus, and Lopho- 

 phoriis ; double in Struthio, Dromceus, 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 taxouomic value from its double condition in Opisthocomus. 

 The researches of Garrod showed that, in the ease of certain 

 notable thigh-muscles, completeness of muscle formula 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. 



Ceratotjlossus. — This is a very strong muscle, arising from the 

 outer side of the ceratohyal, anterior to the (/eniohyoids ; 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. 



Oeratohyoid. — This is a strong wide muscle running from the 

 inner side of the ceratohyal, opposite the insertion of the foregoing 

 muscle to the urohyal. 



The hypoglossah and the system of the sterno-hyoid were 

 present, but the individual muscles were not segmented from each 

 other. 



Depressor mandiJbulcB. — 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 Powls 

 there are at least two portions separable ; in Opisthocomus the 

 tendinous inner portion no doubt represents an inner portion, 



' " On the Anatomy of Palamedea cornuta," P. Z. S. 1894, p. 536. 



