THE SKELETON. 31 



throughout life; or are permanently remote as in Macropus 

 (kangaroo). 



Methods of articulation of inferior maxillary to skull 

 vary in Vertebrata. In Pisces, it is effected by means of a 

 'suspensorium,' which is composed of hyomandibular, sym- 

 plectic,pretympanic, and hypotympanic (quadrate) bones, with 

 opercular appendages (gill covers), in turn composed of oper- 

 culum, pre-operculum, sub-operculum, and inter- operculum. 

 Suspensorium of Batrachia, Reptilia, and Aves, hypotym- 

 panic bone only, the others having disappeared. In Batrachia 

 a peculiar hammer-shaped membrane bone., situated on side 

 of face, may represent the pre-operculum of fishes. (Huxley.) 

 No suspensorium in Mammalia; the articular head of lower 

 jaw convex in this class, in all others concave. 



NATURE OF LIMBS IN VERTEBRATA. 



A vertebrate limb may be defined as an articulated append- 

 age, placed internally to soft parts. 



Pisces. In osseous fishes, anterior extremity united to 

 skull. Exception, Anguilla (eel). Elements of anterior ex- 

 tremity, supra-scapular, scapular, and coracoid bones; hu- 

 merus, radius, and ulna. Distal extremity without phalanges, 

 but furnished with rays of exo-skeleton, thus presenting a 

 fin-like in place of a hand- or foot-like structure. Clavicle 

 and posterior extremity absent. 



Batrachia and Reptilia. No supra-scapular bones; cora- 

 coid bones large, often meeting at median line ; phalanges 

 present. Posterior extremity, with ilium, ischium, and pubis 

 as separate bones. Some extinct Reptilia, tarsal and metatar- 

 sal elements combine. Phalanges present. 



Aves. Anterior extremity, with coracoid bone large, dis- 

 tinct from scapula; clavicle (pull bone, furcula) not joining 

 sternum, but anchylosed with fellow of opposite side ; pha- 

 langes anchylosed to form an acuminate apex. Posterior ex- 

 tremity, with ilium, ischium, pubis, and sacrum anchylosed ; 

 tarsus and metatarsus united to form an elongate shaft, ex- 

 cepting Aptenodytes (penguin). 



