26 rut: AMERIC.W XAICKAI.ISI. ^\ . . XXXV 1 1 1. 



The SivKLEToN IX the L.m.n>. 



In all of its essential characters, the skull of the loons agrees 

 ^vith that of the grebes. In the present description I have 

 chosen the skeletons of Urinator imbcr and Urinator /iiMtne, — 

 the first being the bird known all over the world as the loon, the 

 second, also a largely cosmopolitan type, is the red-throated loon 

 or diver. In the latter the superior osseous mandible is fash- 

 ioned upon the same plan as we found it in the long-billed 

 grebes ; it differs principally in curving very gently upwards, a 

 feature not present in the beak of U. imbcr. The culmen in 

 both species is convex and evenly rounded. Loons have the 

 cranio-facial region depressed, best seen in U. inibcr, where the 

 .sutures between the nasal processes of the preniaxillary and the 

 nasals remain more or less open throughout life. 



All the Urinatoridce are holorhinal, and the dentary process of 

 the nasal dips downwards and forwards in a gentle curve, thus 

 including the' large and somewhat elliptical osseous nares. 

 They are devoid of any osseous nasal septum. Each lacrymal 

 articulates to a very limited extent w4th the corresponding 

 frontal, and almost entirely with the outer margin of the nasal. 

 The OS uncinatum at the inferior apex of its descending limit, 

 fuses with that bone, but the lacrymal does not reach down to 

 the maxillary in U. hnmne, although it often does so in U. imber. 

 In many of the gulls this process is much shorter, and is bent 

 backwards and often anchyloses with the pars plana ; in the 

 Urinatoridse the pars plana is not ossified, and the meseth- 

 moidal plate is thin, showing a raised welt at the usual site of 

 the base of this wing where it occurs in other groups. 



A large vacuity is seen in the middle of the interorbital 

 septum, but the optic and other foramina usually retain their 

 integrity. The track of the olfactory nerve is commonly roofed 

 over for its posterior third or more by an extension of the bony 

 wall which covers the anterior aspect of the rhinencephalon. 

 This arrangement is not seen in a specimen of the skull of 

 Lams glancns, and its interorbital septum is thick and entire. 

 The quadrate has a long, plate-like, and upturned orbital proc- 



