A MONOGRAPH OF THE BATS OF NORTH AMERICA. 29 



III. Tho Bird, iu which tho hiimeruH is curved, more or less slender; pectoral 

 ridge prominent, not angular; ulna large, curved, not united with the 

 slender and more dinunutive radius; carpus or two l)ones; metacarjius of 

 two, sometimes of three bones, the lirst l)eing snuill and cylindrical, the 

 other two of larger dimensions and united so as to form a bone resembling 

 those of the forearm ; ulnar ])halaux of one joint, united to the radial, 

 which is composed of two. 



Thepower of sustaining tlight not dependent upon the expansion of skin, 

 but upon the excessive development of dermal appendages (feathers). 



IV. Tho Archaiopteryx agrees with the typical bird iu general i)articulars, but 

 differs in the number of metacarpal bones, which are here four in number : 

 the first and second are slender, free and separate from one another; the 

 third and fourth bear considerable resemblance to those of extant birds, in 

 beiuglarge, stout, and closely approximated ; but are not, however, united. 



Flight is supposed to have been maintained in the same manner as in 

 living birds. 



In addition to tlie instances already given, certain fislies, as the 

 Exocoetm and Bacfylopterm, -possess the power of snstaining true flight. 

 The mechanism that lifts the body of the flsli from the water, and nj)- 

 holds it for a short time in the air, is obtained in the pectoral fins 

 which, in these animals, are enormously developed. The structure of 

 these fins is homologous to that of the anterior extremities of other 

 vertebrates, their form alone being modified to adapt the animal to 

 the medium in which it is placed. Thus we have, in each great subdi- 

 vision of vertebrate animals, a representative capable of sustaining 

 flight. 



Another somewhat similar modification of the animal economy is met 

 with in a few animals of arboreal habits. Here a peculiar arrange- 

 ment of the skin is observed, which enables the possessor to break the 

 force of downward leaps. In the Flying Lemur {Galeopithcctis), in the 

 Flying Squirrel (Pteromys), and in the Flying Opossum {Petaiirista), 

 the furred skin extends laterally from the sides of the body and is 

 attached to anterior and posterior extremities at the metacarpal and 

 metatarsal regions respectively. The only instance of osteological 

 development is obtained in the Dragon {Draco vohois), a small lizard 

 from Sumatra, in which long, transverse processes from either side of the 

 lumbar vertebrae support a thin membranous growth which is capable 

 of being opened and shut by means of muscles attached to the bony 

 frame-work. 



tp:eth. 



In describing the molars the nomenclature of H. F. Osborn will be 

 followed. The diagram lierewith presented is copied from this writer's 

 paper in the American i^aturalist, December, 1888, p. 1072. 



UPPER MOLARS. 



Antero-internal cusp Protocone. pr. 



Postero-internal cusp or sixth cusp llypocone. hy. 



Antero-external cusp Paracoue. pa. 



Postero-exterual cusp Metacone. me. 



Anterior intermediate cusp Protocouule. pi. 



Posterior intermediate cusp Metaconule. lal. 



