STERCORAEIUS POMATORHINUS, POMARINE JAEGER. 609 



bellied birds are males, the others females. This view is combatted by most ornithol- 

 ogists; and some even of those who admit that the difference in color is a sexual 

 peculiarity, say that it is the males and not the females that are dusky all over. This 

 diversity of opinion I consider as strong evidence that soc does not determine the plum- 

 age. I think that the four "varieties" which Temminck characterises are really the 

 progressive stages of the same individual— stages which every Jaeger must pass through 

 or attain to in arriving at maturity. The birds which are in the unicolor state, so dif- 

 fereut from that of the adults, all have weaker bills, less elongated central rectrices, 

 particolored titrsi, and are generally smaller and less robust than the adults, all of which 

 points indicate immaturity. As regards the other conditions, the fact of our being 

 able to trace one directly from another, seems to me proof that they are not permanent 

 during the life of any single individual. (See continuation of the discussion of this 

 question under head of S. ■parasitioaa.) 



Anatomical characters. — The mouth is capacious. It is quite narrow at the tip of the 

 bill, but by the rapid divergence of the rami of the mandibles, quickly becomes very 

 broad. The palate is not arched nor vaulted at all, but quite flat ; but the integu- 

 ment, and membranes forming the floor of the mouth are very loose, and capable of 

 considerable distension. The membrane at the angle of the mouth is loose and full ; 

 and a longitudinal elevation of it on the lower mandible is received into a correspond- 

 ing depression in the upper. 



The palate is quite soft and easily movable. On it there is a central very prominent 

 longitudinal ridge, which commences at the very apex of the bill. There it is exceed- 

 ingly narrow, but it widens and becomes less prominent as it goes backward. About 

 half way to the nasal aperture there are a few reversed papillas. Somewhat less thani 

 an inch and a half from the tip of the bill this ridge bifurcates, being longitudinally 

 divided by the slit forming the opening of the uares. The edges of this slit, accurately 

 coaptated, are about three-fourths of an inch long, and beset with reversed acute pa- 

 pilla}, which are especially large at the posterior extremity of the sides of the slit. By 

 the approximation of these straight edges the nasal aperture can be perfectly closed ; 

 but posterior to where they terminate, the nares remain permanently open. 



Ou either side of this central ridge is another, quite high and prominent, which, 

 however, subsides an inch from the tip of the bill. Posteriorly this ridge also is beset 

 ■with line reversed setiB. The space between this and the median ridge is quite smooth 

 and flat for some distance further back, where it is terminated by a single fringe of 

 long, slender papillae. 



The membrane of the floor of the mouth is loose, flaccid, and, in the nndistended 

 state, longitudinally wrinkled. The tongue is stout ; tapers but little, and is fleshy 

 quite to the tip itself, which is scarcely at all horny, but bifid. The dorsum of the 

 tongue is longitudinally channelled for its entire length ; its sharp sides rising up from 

 the longitudinal central depression. The base is square and terminated by a fringe of 

 papillae pointing outward and backward. The tip is quite deeply notched, the base 

 emarginate, with a lateral projecting papillated cornu on either side. 



About half an inch behind the base of the tongue the rima glottidis is found. As 

 usual, it is a simple slit ; about half an inch liTng in this species. The elevation over 

 the larynx, in the centre of which the glottis opens, is on its posterior hiilf, thickly be- 

 set with stout, rapidly tapering, conical papilla). 



The basi-hyal element of the hyoid bone is about a third of an inch long ; flattened, 

 subrectangular, tapering anteriorly to an obtuse, nodulated, deflected extremity, which 

 supports the glosso-hyals. It articulates moveably with the glosso- and apo-hyals, im- 

 moveably with the uro-hyal. The glosso-hyal elements are chiefly cartilaginous ; they 

 lie side by side imbedded in the substance of the tongue, being articulated with the 

 hasi-hyal by their under surfaces, so that their posterior extremities project backward 

 beyond their articulation, as two prominent cornua, which support the posterior ex- 

 tremity of the tongue. The uro-hyal is consolidated with the basi-hyal. It is osseous 

 for about a fourth of an inch, when it becomes cartilaginous and tapers gradually to a 

 very acute apex. 



The apo-hyals are about one and a third incljes long. They are nearly straight, en- 

 tirely osseous, cylindrical,e njoying free motion at their basi-hyal articulation. From 

 their extremities the cerato-hyals, chiefly cartilaginous, curve upward and outward for 

 about an inch. 



The (esophagus measures about ten inches from the glottis to the cardiac oriiice of the 

 gizzard. It is very capacious, and capable of being exceedingly dilated. In the un- 

 distended state the lining mucous membrane is thrown up into numerous very prom- 

 inent longitudinal rugte, which of course disappear upon dilatation. These rugae are 

 perfectly straight in the lower three-fourths of the canal, but toward the fauces they 

 become wavy and zigzag. The contractile power of the tube is very great, as is evi- 

 denced by the thickness of the planes of muscular fibres by which it is enveloped. 

 These fibres are both longitudinal and circular, and increase in volume toward the pro- 

 ventriculus. 



The normal diameter of the canal seems to vary but slightly throughout its whole 



39 



