1886.] GEOCOCCYX CALIFORNIANUS. 469 



The spinal pterylosis is represented by two broad, longitudinal 

 tracts well separated from each other in the middle line by a naked- 

 skin area overlying the vertebral column. These spinal pterylse 

 gradually converge towards each other as they approach the pelvic 

 region, over which they merge into one feather-space, which becomes 

 pointed behind and terminates just in front of the nude oil- 

 gland. 



The upperside of the coccygeal protuberance is also sparsely 

 feathered, being divided from a more generously covered area below 

 by the line of horizontally arranged pits for the quill- butts of the 

 rectrices of the tail. An exceedingly narrow tract, on either side, 

 springs from the posterior margin of the outer bifurcation of the 

 ventral tract just below the arm-pit, to run longitudinally down the 

 side, in front of the thigh, to become lost before it arrives at the 

 margin of the vent. This strip seems to be composed simply of a 

 double row of feathers, and might appropriately be termed the 

 lateral tract. 



The patagium of the arm is very thinly feathered anteriorly, 

 while its dorsal aspect is quite completely covered ; the remainder of 

 the pterylosis of the pectoral limb presents us with nothing of a 

 peculiar nature. The posterior marginal boundary of this " alar 

 tract" is, as usual, terminated by the row of quill-butts of the 

 primary feathers of the wing. 



Passing now to the crural region, we find a posterior limb but 

 sparsely feathered, while a conspicuous " femoral tract " extends 

 obliquely across the dorsal aspect of the thigh, and rapidly narrow- 

 ing, runs along the pubic line, finally merging into the " caudal 

 tract." 



Geococcyx shows its best-marked apteria in front, in the mesial 

 space between the inner strips of the ventral tracts, and laterally to 

 the outer side of the spinal pterylosis, where, indeed, for a con- 

 siderable space no feathers may be said to occur at all. A. well- 

 marked dorso-longitudinal naked area is also to be seen. In this 

 Ground-Cuckoo we notice a membrane, stretching between the 

 thigh and leg, which corresponds to the patagial fold of the pectoral 

 limb, and fully as well-developed. 



Before closing this part of our subject it is of interest to observe 

 the general form of Geococcyx, which, now that its feathers have 

 been removed, can be studied to the best advantage. It will be seen 

 how perfectly its figure has come to assume a shape best adapted to 

 the peculiar requirements of the bird. Its pelvic limbs are large, 

 muscular, and consequently powerful, while the pectoral ones are 

 decidedly less so, though by no means weak or inefficient. The 

 body-form of this prince of avian racers is what almost might be 

 called " clipper built," so admirably fashioned is it to the needs and 

 ends of a rapid running bird of the size of our subject. 



With these few remarks upon the external organization and 

 appearance of Geococcyx we will now close this part of our discussion, 

 and pass to the consideration of some of the features presented in 

 the structure of the remainder of its economy. 



