EVOLUTION OF PENGUINS. 25 



petrels, however, in so far as I have been enabled to study skulls of the necessary age, 

 ditt'er conspicuously in that the squamosal has assumed au arcuate form, the upper limb 

 thereof extending along the iufero-lateral border of the frontal. This change in the 

 form and relations of the squamosal, by which it comes into combination with the 

 frontal, is met with in the more highly specialised types of many quite unrelated orders 

 of birds, though the details of the union vary. It is associated generally with an increase 

 in the size of the frontal, and a decrease in that of the parietal, but the net result appears 

 to be a larger cranial cavity. The peculiar form of the squamosal in the Tubinares, 

 it is curious to note, occurs nowhere else except among the Alcidse — the most highly 

 specialised of the plovers ( Charadril). 



The Steganopodes have lost the external nares, excepting only the Phsethontidse, 

 in which, however, they are much reduced. The Sphenisci have similarly suppressed 

 the external nares completely, except in Spheniscus, in which, however, they have 

 ceased to Ije functional. The manner of this sealing up of the nares is interesting. 

 In the penguins, this closing has been brought about simply by the growing 

 together of the rhamphothecal horny tissue surrounding the aperture, leaving 

 the osseous nares unaffected. In Spheniscus, however, a minute though function- 

 less aperture still remains, yet here the osseous nares have also begun to close. 

 Among the Steganopodes, the cormorants and gannets have completely closed the 

 osseous narial aperture by Ijony tissue, and show Ijut the faintest traces of an aperture 

 in the rhamphotheca ; in the pelicans there is no trace externally of these apertures, 

 and only minute apertures in the place of the normal fossae in the skull. The process 

 of reduction of the osseous fossa is interesting, inasmuch as in the nestlings of the 

 Ph8ethontid8e,for example, the skull is schizorhinal,while in the adult it becomes holorhinal. 

 This is true also of the cormorants and gannets, though here the fossa becomes almost 

 obliterated. This reduction of the nares is the more remarkable because in the Colymbi, 

 which, like the penguins, o])tain all their food by diving, the nostrils are normal. The 

 Tubinares show a similar reduction of the nares, but in no case does this end in complete 

 suppression of the aperture. If we turn to the Ciconise, Anseres, and Accipitres we find 

 the same phenomena ; the more primitive types have the largest nares. Thus, then, 

 this peculiarity appears to have been the common heritage of all the forms Ijelonging to 

 this great Steganopod branch of the Avian tree, except the Colymbi. In their pterylosis 

 the penguins are the most primitive of all the Neognathse (Carinatse), and after them the 

 Steganopodes show the broadest pteryla. 



Dr. Chalmers Mitchell in his extremely valuable treatise on the Intestinal Convo- 

 lutions of Birds treated the characters of the patterns of the gut as though this portion of 

 the alimentary canal were the whole animal. Although, as he pointed out, the evolu- 

 tionary inferences therefrom cannot be regarded as exact indications of affinity between 

 the different groups of birds, yet they follow surprisingly close on the results obtained 

 by taking the sum of all the anatomical characters. In so far as it bears on the 

 present paper, then, we may remark that he showed that the characters of the intestual 



