126 PROP. OWEN ON AEGILLOENIS EONGIPEXNIS. 



panded surface so overhung the fossil is remarkable. The albatross, 

 again, most nearly resembles it in this character, but without the 

 overhanging production of the head (compare figs. 1 & 5). The 

 smooth outer crust of this surface is impressed in the fossils with 

 minute points, and shows very faint longitudinal striae. 



There is a trace of the foramen pneumaticum (fig. 2, /) at the 

 bottom of the fossa beneath the ulnar tuberosity, the walls of the fossa 

 being broken avray. 



The thinness of the compact outer wall of the shaft (with the 

 concomitant wide air-cavity), and the size and conformation of this 

 part of the wing-bone, bespeak a bird of flight, and a species as 

 large, at least, as any of the existing birds which enjoy the charac- 

 teristic locomotion of the class. 



The portion of the right humerus includes, with the head (fig. 3) 

 and indications of proximal structures above described, five inches 

 of the shaft. Prom a preserved breadth of 2 inches it diminishes 

 to that of 10 lines at the fractured end. The smooth ridge noted in 

 the left humerus (fig. 2, h) is continued distad for 2\ inches before 

 subsiding. 



With the head of the left humerus were brought three portions 

 of a long limb-bone with the pneumatic structure of the humerus 

 in longipennate birds of flight (PL YI. figs. 9 & 10), and corre- 

 sponding in size with the portion of the shaft of the right humerus, 

 but less crushed and distorted. 



The first of these portions I infer to be from the proximal half of 

 the shaft ; it is three-sided, with the angles broadly rounded off, but 

 least so at that angle which shows the base of an outstanding ridge. 

 An inch and a half of this base or origin is preserved (figs. 7 & 8, i), 

 continued from the proximal fractured end of the bone. I infer it 

 to be part of the origin of the " pectoral crest." Anconad of this 

 origin, and about 3 lines distant therefrom, is a linear ridge (fig. 7, h) 

 running longitudinally parallel therewith, and bending slightly more 

 anconad before terminating at the distal fractured end of the por- 

 tion. This ridge denotes the insertion of the ancono-deltoideus, or 

 " posterior deltoid muscle." 



Now, as to the trihedral shape of this proximal portion of the 

 humerus, which may include one fourth or one third of the shaft, 

 such shape is not shown by the humerus in the majority of birds : 

 I find it in certain Eaptores, Longipennates, and Totipalmates. In 

 all that do show it one of the sides is palmad, the other two sides 

 are anconad (as in figs. 7, 13). In the eagles and vultures (Vultur 

 monacJius, ]nx>nogr. cit. pi. iii. fig. 7) the angle dividing the radial 

 from the ulnar sides of the anconal surface is sharper than in 

 Argillornis. 



Pelecanus and Diomedea come nearest to the fossil in the obtuse- 

 ness or degree of rounding-off of that angle, but fall short of that 

 degree shown in the fossil. In all the characters above compared, 

 the portion of shaft (fig. 7), like the head of the bone (PL YI. fig. 1), 

 is from a left humerus. 



The next character which I find available in steering toward 



