OF COLYMBUS TORQUATUS. 



161 



the bird in a fresh state, we find, on closing the anterior toes, that they do not lie all beside 

 each other ; but that the inner one lies almost directly behind and between the other two. 

 This arrangement of the toes is in order that the least possible resistance to the water shall 

 be affordecf by the toes when they are closed in the forward motion of the feet. To pro- 

 duce this effect, the inner of the three phalangeal heads of the bone is given off before the 

 other two, and not by the side of them ; but so tar around on the inside of the meta- 

 tarsus, towards its posterior aspect, that it lies almost directly beneath and behind the mid- 

 dle head On looking at the bone directly from before, this head is quite hidden from view. 

 In consequence of this arrangement, the line of the articulation of the inner toe is very 

 oblique to enable the toe to be extended without interfering with the one next to it. 



The two other heads which form the distal extremity of the metatarsus, are placed side 

 by side ; the middle one being a little the larger and longer. They are very convex 

 vertically and each is marked with a median groove for the reception of corresponding 

 median ridges on the heads of the phalanges. They are separated from each other by an 

 exceedingly narrow but very deep furrow. _ 



Just above the inner head of the bone, on its interno-postenor edge, is a slight oval 

 depression on which rests the accessory metatarsal bone. This is a small, thin, flat, falci- 

 form almost unciform lamina of bone, entirely disconnected with the main bone, and simply 

 applied loosely against it. It has no real articulation with the metatarsus ; but its distal 

 extremity is thickened for the support of the basal phalanx of the hallux. 1 



i I have recently been much interested in examining the very loosely connected with the metatarsus, and again present, 



very diverse modifications of this os metatarsal accessorium, ing the appearance of a third phalanx, it being hall as long 



both as regards its shape and its mode of connection with the as the real basal phalanx. In the anomalous genus Rissa, 



metatarsus" proper. To the end of ascertaining the amount usually described as « tridaetyle," both this bone and the proper 



of its variations in these respects, I have examined the halluces basal phalangeal segment, are present and quite distinct ; the 



of many families and genera; in which labor I have been ungual segment being the deficient one 



areatlv assisted by my friend, Mr. A. E. Verrill, of Cambridge, There is a remarkable peculiarity in the hallux of J iiffmus, 



Mass ' who has taken the trouble to examine many forms Thalassidroma, and probably most of the other genera of the 



which I had not at hand. The result of our observations large family Probanda. This is, that while the accessory 



shows that throughout the Natatores, and to a less extent metatarsal is present the true basal phal anx.s wanting 



amon „ ot her Aauaticce, the bone deviates most remarkably Professor Owen, indeed, says (Todd s Cyclop., p. 288) that 



from hie ordinary type, as presented by the higher orders of "In the Petrel, however this accessory metatarsal bone 



Raptores, Insessoresjc. Instead of being very intimately con- is wanting, although the hallux ,s present ; the two bones of 



nected with the metatarsus, or even more or less completely an- which, are therefore united to the principal metatarsal bone 



chylosed with it, and consequently immovable, and of a shape by long ligaments." But, as suggested by Professor Wyman 



and general appearance which unmistakably point to its true it seems most rational that in such doubtful cases as the present, 



character this metatarsal element becomes totally disconnected we should be guided in formmg our opinions chiefly by the 



with the metatarsus except ligamentously, and is therefore shape and position of the bone ; and that, too, even if a cap- 



cuite loose and freely movable ; in some instances there is a sule and synovial membrane exist, as is probably the case in 



smooth articulating surface, which may not impossibly be even Graculus. The bone in the genera now under consideration, 



lined with synovial membrane, while a frequent elongated and has much the characters and general appearances which obtain 



cylindroid shape of the bone itself, joined with its other char- throughout the allied family Laridcc ; which would seem to 



acteristics, gives it the appearance rather of a third phalangeal indicate correctly its real nature. 



merit '° The whole subject of the number of phalanges and their 



"Tappears to be among the Totipalmi that the bone receives peculiarities in the class Ave,, is an extremely ^teratmg 



its most peculiar modification in size, shape, and connection, one, and well worthy of extended mvest.gation. Nn e the 



In Sula,PeI,canus, Graculus, &c, this metatarsal bone is announcement by Cuvier of the normal number ot P^g* 



nearly as Ion* as the basal segment of the hallux, slightly and of phalangeal segments among birds, the matte, .Iocs not 



curved, and of a cylindroid shape Its proximal extremity has seem to have received the attention it merits, perhaps on ac- 



„ the' perfect articulating surface which its distal end bears-, count of the well-known remarkable *£f^*SZ 



but nevertheless it is smooth, and nearly gives the impression which obtains throughout the class. But I am of opinion that 



of an articulating facet. Its haracter as a metatarsal element concerning this particular point there probably remain many 



Areatlv over-balanced by its phalangeal features, so that it anomalies, or at least peculiarities y* to be brought into notice. 



SybJ analog that we can determine its proper place. Writers have not generally in their tables given, by any means, 



Throughout the Lane**, this bone is found" as a small, the whole number of the exceptions to the general rule, that are 



flattened, rounded, or irregularly polyhedral osseous nodule, now well known to exist. 



