PLEXUSES. 287 



that in ventral aspect, the latter appears to lie in a transverse depression of 

 the large lobe. The left lung is simple, but has a slight cleft on the ventral 

 margin anteriorly. A large azygos lobe lies transversely just posterior to the 

 heart. Its shape is pyramidal, with a triangular cross section; its base lies 

 near the middle of the right lung and it tapers to a point on the left side, where 

 it is partly received into a depression of the ventral surface of the left lung. 

 Apparently the condition is much the same in the Echidna, though Westling 

 (1889) states that in this animal the right lung is three-lobed, the left single. 

 Apparently the azygos lobe was mcluded with the two lobes of the right lung. 



PLEXUSES. 



The plexuses of the Echidna have been studied and carefully figured by 

 Westling (1889) and those of the Proechidna appear to differ but little. The 

 brachial plexus is composed of the same nerve trunks in both animals. In the 

 Proechidna, cervical nerves 7 and 8 unite almost at once to form a common 

 trunk. The fifth and sixth cervicals are about equally large and unite with 

 the combined seventh and eighth at about 17 mm. from the exit. Cervicals 

 3 and 4 are slightly smaller and the former unites with a very small second 

 cervical before joming the other, chief trunk. 



The lumbosacral plexus is formed by nerves from the last dorsal, the four 

 lumbar, and two anterior sacral vertebrae. In the Echidna, Westlmg figures 

 this plexus from an animal having 15 and another having 16 dorsal vertebrae. 

 In the first, the plexus consisted of seven trunks, one from each of the last two 

 dorsals, the three lumbars and two anterior sacrals; in the second it has an 

 additional element from the antepenultimate dorsal, but in each case the first 

 nerve to enter the complex was that of the 14th dorsal, indicating in the case 

 of the specimen with but 15 dorsals that the 16th had been lost. Thus there 

 is the same nmnber of nerves in the plexus in both anmials, notwithstanding 

 that the Proechidna has an additional lumbar. The difference lies in the fact 

 that only one dorsal enters the complex in the Proechidna. The fact that there 

 are but two sacral nerves to enter the plexus in both the Echidna and the Pro- 

 echidna lends weight to the contention of Gregory' (1910, p. 152) that the num- 

 ber of true sacrals in both genera is two, and that the third or last sacral is 

 really a modified caudal. On the other hand the three vertebrae usually con- 

 sidered sacrals have theu- transverse processes entirely united with the dorsal 

 edge of the iUum and in this respect appear together to constitute the sacrum. 



