MONOGRAPH OF THE PETRELS. 



in these pages should consult the splendid Monograph of W. A. Forbes ("Challenger" 

 Reports, Vol. IV., Part XI.), or the various treatises quoted in the following pages. 



External (Exoskeletal) Characters. 



In the matter of the pterylosis of the Petrels much work remains to be done. 

 The little that has been published on this theme is chiefly remarkable for its 

 inaccuracy. All who have written on the subject seem, as usual, to have derived 

 their information from Nitzsch, who, having many difficulties to contend with in the 

 way of obtaining fresh material, was frequently wrong in his interpretation. 



Without entering into a detailed description of any particular species it will suffice 

 to mention the principal peculiarities of the several tracts in a few widely distinct 

 species. 



In the smaller and, I presume, the more primitive types, the spinal tract 

 is very narrow, and is traversed by a linear ephippal space extending from the 

 interscapular region to the level of the middle region of the pre-ilium. The ventral 

 tract is also narrow, and passes on each side, in the abdominal region, into a very 

 narrow band of feathers enclosing a large apterium mesogastrcei. 



It would seem that in all the other Petrels belonging to the Procellariince, in 

 the Pelecanoidince, and in the Diomedeidce, the tracts, both dorsal and ventral, were 

 conspicuously broad, the spinal tract passing backwards, on either side, to blend with 

 the femoral tract. In Puffinus anglorum the spinal tract is solid — that is to say there 

 is no ephippal space. In Prion and Pelecanoides, on the other hand, and probably 

 in the majority of genera, this space is fairly well marked. In Pelecanoides the tract 

 on each side of this space is excessively broad. 



In the Albatroses the spinal tract has always been described as interrupted, as 

 being forked caudad of the scapulae, the rami of the fork embracing the fore end of 

 the median hinder moiety of the tract. This is not so. In Diomedea exulans, for 

 example, the spinal tract is continuous, extremely broad, and embraces a very narrow 

 ephippal space. Caudad the tract fuses with the femoral tract. The ventral tracts 

 are broad, and the median abdominal space (apterium mesogastro&i) is very narrow. 



The wing in all Petrels is diastataxic or "aquintocubital." Owing to differences 

 in the relative lengths of the outermost primaries, the contour of the wing varies, but 

 in all the Petrels, apparently, the secondaries are very short, not greatly exceeding their 

 major coverts in length. The intermost primary is reduced to the condition of a 

 remicle, much shorter than its covert. In all there are 11 primaries; while the 

 secondaries range from 13 to 40, the latter obtaining only in the Albatroses. The 

 carpal remex and carpal covert are large. The major coverts of the hand are large, 

 those of the secondaries small, though from the narrowness of the remiges they appear 



XVI 



