324 THE NICOBAR PIGEON. 



three-fourths of an inch in diaineter. The outer or convex surface 

 of each cartilaginoas body fits into a cup-shaped cavity which is 

 lined by a semi-cartilaginous membrane, the whole constituting a 

 ** ball-and-socket " joint with well lubricated surfaces. The two 

 surfaces of this pseudo-articulation are capable of easy movement 

 on each other, being retained in close apposition by the attachment 

 to the subjacent tissues of the horny epithelial lining membrane in 

 which the cartilaginous body is developed. The inner or free sur- 

 face of each hemispherical body, that which looks into the gizzard 

 cavity, is somewhat concave, and projects a little above the surface 

 of the lining membrane ; it is much harder than the opposite con- 

 vex side of the cartilage and has almost the consistence of bone, 

 the arrangement of the cells into densely packed rows with but 

 little intervening matrix indicating an approach towards ossification. 



The firm consistence of these hemispherical cartilages combined 

 with the mechanism of a moveable articulation must greatly assist 

 the already powerful muscular walls of the gizzard ; but there is an 

 additional factor in the crushing power in the constant presence of a 

 small quartz pebble, usually about half-an-inch across. With such a 

 apparatus, I can well conceive that very hard seeds and nuts may 

 be broken, as in the case of the seeds of Adevanthera pavonina 

 already alluded to. The Nicobar j^igeon is in fact possessed of a nut- 

 cracking mechanism in its gizzard, by which nuts like those of our 

 hazel tree would bo cracked with comparative ease. 



With reference to the small quartz pebbles found in the gizzards 

 of these birds, I should remark that there is usually only one 

 present, and that it varies in weight between 30 and GO grains. I 

 was sometimes able to say where the pigeon had obtained its 

 pebble. Thus, in Faro Island the bird often selects one of the 

 bipyramidal quartz crystals, which occur in quantities in the beds of 

 the streams in the northern part of the island, where they have been 

 washed out of the quartz-porphyry of the district. In other in- 

 stances the pebble seems to have been originally a small fragment of 

 chalcedonic quartz, such as composes some of the flakes and worked 

 flints that are found iflf^ the soil which has been disturbed for 

 cultivation. Sometimes the pebble is of greasy quartz ; and now 

 and then in the absence of quartz the bird has chosen a pebble of 

 some hard volcanic rock. It is a singular circumstance that 

 although these pigeons frequent coral islets where they can easily 

 find hard pebbles of coral-rock, they prefer the quartz pebbles which 



