NATURAL SCIENCES OF PHILADELPHIA. 135 



progress towards maturity. The only question is, what age is to be assigned 

 to this dusky state, and whether one or both sexes participate in it. 



Leaving now speculation for facts, we have yet to notice the mature bird. 

 Taking either the dusky state (No. 20302,) or No. 2754, we find that both tend 

 to produce the following plumage : 



Nearly mature, (No. 20144.) — Size and form of the adult. Pileum and 

 latero-nuchal region, and whole upper parts, as in the adult. The under parts 

 white (as in the adult), but clouded everywhere with dusky patches, most 

 marked across the breast, on the sides, the flanks, and under tail coverts, and 

 leaving the middle of the belly and throat nearly pure. Varying degrees of 

 this dusky nubilation approach in some specimens nearly to the uniform dusky 

 above characterized ; in others fade almost into the pure white of the adult, 

 — connecting the two ages perfectly and uninterruptedly. The tarsi of those 

 specimens most dusky have small yellow blotches ; the others not. 



Now, by the complete obliteration of these dusky cloudings on the throat, 

 breast and belly, and its increased intensity on the under tail coverts and 

 abdomen as far as the flanks, we arrive at last at the 



Adult, perfect plumage, {1^0. 10802). — Bill a little shorter than the head or 

 tarsus, about equal to middle toe without the claw; stout, about as high as 

 broad at the base. Cere longer than the nail. Culmeu broad and flattened, 

 with a longitudinal groove on each side. Nostrils as in the other species. 

 Curvature of culmen and tomia very gradual. Gonys short, about straight; 

 rami very long, a little concave ; eminentia symphysis little marked. Stride 

 and sulci as in the other species. Tarsi about as long as the middle toe and 

 claw, moderately stout, somewhat roughened supero-posteriorly, but not 

 nearly as much so as in pomarinus. Scutellation and reticulation of tarsi, toes, 

 and interdigital membranes as in the other species. Tail moderately long, 

 slightly graduated, the lateral feathers broad quite to their apices, which are 

 somewhat truncated, the shaft slightly protruding as a small mucro ; the central 

 pair projecting three to four inches ; rigid ; not losing much of their breadth 

 until about four inches from their ends, when they commence to converge 

 regularly to a quite acute apex. They have nothing of the filamentous char- 

 acter of those of Buffoni. Wings long, powerful, their rhachides rigid, their 

 apices somewhat acute. Pileum, occipital crest, whole upper parts, deep 

 brownish black, with a somewhat slaty tinge, and a slight but appreciable 

 metallic nuance; this color deepening into quite black on the wings and tail. 

 Rhachides of primaries and rectrices whitisla, except at their tips ; the inner 

 vanes albescent baso-internally. Chin, throat, sides of head, neck all round 

 and under parts to the vent, pure white ; the feathers of the latero-nuchal 

 region rigid, acuminate, with disconnected fibrilla;, light yellow. Under tail 

 coverts like the upper parts, but somewhat of a fuliginous tint ; the line of 

 demarcation from the white of the abdomen very trenchant. 



Dimensions of fully adult. — Bill above 1-40 inches ; height or width at base, 

 about -50. Wing, ifrom flexure, 1300: tarsus, or middle toe and claw, 1-80 ; 

 tail 5J, its centre feathers nearly 9-00. 



Dimensions of young of year. — Bill above 1-30 ; height or width at base -40; 

 wing 12-00 ; tarsi, or middle toe and claw, 1-TO ; tail 5-00 ; central tail 

 feathers not quite 0-00. 



Steecorarius Richardsoni Coues ex Swainson. 

 Lestris Richardsoni, Swainson, Fauna Boreali-America, 1831, ii. p. 433, pi. 

 ixxiii. sed non auctorum. 

 Habitat. — Interior of Arctic America. 



DiAG. — S. Stercorario parasitica similis ; sed major, rostro, tarsis, alisque 

 longioribus ; cauda magis producta et rotundata, rectricibus latioribus. 

 This species is treated of at length in the beginning of the present paper. 



1863.] 



