44 THE VOYAGE OF H.M.S. CHALLENGER. 



Tlie Tarsus. 

 The tarsal bones, as shown by Gervais and Alix ^ in their observations on the anatomy 

 of the embryonic Penguin, present the same peculiarities as in birds in general — one 

 becoming coalescent with the lower end of the tibia, the other with the upper extremity 

 of the metatarsus. 



The Metatarsus. 



The metatarsus (PI. VII. fig. 11) of the adult Penguin differs from that of other 

 birds, inasmuch as it shows clearly its original composition through the union of four 

 distinct metatarsal bones. 



The first or innermost metatarsal is very small, and almost rudimental. It is not 

 anchylosed with the others, but articulates by its base with the lower end of the shaft of 

 the second metatarsal bone by means of a moveable joint. 



The second, third, and fourth metatarsal bones are anchylosed together to form 

 a single osseous mass, with which, moreover, the lower tarsal element as in other 

 birds is inseparably united. The original composition of the mass by the union of 

 three distinct metatarsal bones is clearly indicated on the anterior surface of the bone 

 by the presence of two well-defined longitudinal grooves. These grooves are deeper at 

 the upper than at the lower end of the bone, and the lower portion of the external 

 is prolonged slightly beyond that of the internal groove. The upper end of each 

 groove, at least in Eudyj^tes chrysocome from Tristan d'Acimha, ig perforated by a 

 foramen which appears on the posterior surface of the bone. Through these inter- 

 metatarsal foramina, minute blood-vessels pass from the front to the back of the limb. 

 On the posterior surface of the metatarsus, the position of the intermetatarsal grooves 

 is indicated only by these foramina. With regard to the dimensions of the three outer 

 metatarsal bones, the second and fourth are nearly of equal length, the second slightly 

 longer than the fourth, while the third is longer than either. The second metatarsal 

 bone is more slender than either the third or fourth, both of which are of nearly equal 

 thickness. The shaft of the third metatarsal bone is, moreover, provided with a small 

 tubercle situated opposite the junction of the upper and middle thirds of its length for 

 the insertion of the tendon of the tibialis anticus muscle. The lower extremities of the 

 three larger metatarsal bones are quite distinct, being separated by notches which, even 

 more distinctly than the intermetatarsal grooves themselves, indicate the original sepa- 

 ration of the component elements of the compound metatarsus. The shafts of the third 

 and fourth metatarsal bones are nearly parallel with one another from end to end, but the 

 second metatarsal bone diverges considerably at the lower end from the third. In conse- 

 quence of this arrangement, the metatarsus of Eudyptes is somewhat broader at its lower 

 than at its upper end — a point in which the members of that genus difl'er somewhat from 

 those of the others which I have examined. The lower extremities of the metatarsal 



Osteologie des Mancliots, pi. xvi. fig. 8. 



