SHUFELDT : OSTEOLOGY OF THE STEGANOPODES 145 



The posterior border of this lateral aspect shows a well-marked ilio-ischiadic notch 

 at about the middle of its extent. This character is best marked in S. cyanoi:>s and 

 8. gossi, being much shallower in other species, especially in S. piscator. The outer 

 side of the bone between it and the ischiadic foramen is directed upward as well as 

 outward. 



As has already been mentioned above, there are usually seven free vertebrae plus a 

 large pygostyle in the skeleton of the tail of most Gannets and Boobies, but an 

 exception is noted in the case of Sala cyanops, in which species I count in the 

 specimen before me, only six and the pygostyle. The characters of these vertebrse 

 may be well studied in the skeleton of the tail in Sulci brewsteri. What first strikes 

 one upon glancing at this part of the osseous system of any Gannet is its compara- 

 tive massiveness, the great size of the individual vertebrae, and the large pygostyle. 

 The neural spines however, are short and stumpy; in Sula bassana they are occasion- 

 ally bifid anteriorly. The neural arches beneath them close over the spinal canal for 

 the entire length of the series, and the latter, for a short distance, is seen to perforate 

 the pygostyle. The transverse processes are thick and strong, being very wide-spread- 

 ing, especially in the case of the ultimate and penultimate free ones. From four to 

 five of the last ones usually develop h£emal spines. These become larger as we 

 approach the pygostyle, the first vertebra of which also has one, it being bifid, 

 and hooking forward, as do those on the caudals in front of it. The interarticular 

 facet on the faces of the centra are nearly flat, showing barely any concavity or 

 convexity. 



The pygostyle appears to be composed of about six vertebrae, of which the three 

 anterior ones can be quite easily made out. It has a very unusual form in this bird, 

 being very long and subconical, with sharp superior border and rather decurved 

 apex. Below, it is broad and somewhat convex. Viewing it from in front we notice 

 that it has all the elements present, though in very rudimentary state, of one of the 

 caudal vertebrae, including the large, prominent and anchylosed haemal spine just 

 mentioned. 



The Appendicular Skeleton. 



The Pectoral Limb. — We find the humerus in Sula bassana to be somewhat longer 

 than the radius and ulna in this limb, but we shall see later on that this varies 

 greatly in the other species. I will write out here first an account of the appendic- 

 ular skeleton of Sula bassana, and close with the differences exemplified on the 

 part of the other Gannets and Boobies we have under consideration. In this 

 humerus the ulnar crest is prominent and projecting, though rather inclined to retreat 

 from the elongated and shallow pneumatic fossa that arches over it, as in many other 



