96 SPECIAL CONVERGENCE 



reciprocally homologous, whilst others, like the 

 above, are homoplastic. 



Another case worth quoting, as well known to 

 students of osteology as it is remarkable, is that 

 of convergent opisthocoely already mentioned by 

 Owen l as occurring in the North American fresh- 

 water Ganoid fish, Lepidostens the Garpike, in 

 the Surinam toad, Pipa americana, and in the 

 Salamanders. In Batrachia and Reptilia the 

 vertebrae articulate together by a cup-and-ball 

 or ball-and-socket arrangement, and, as a general 

 rule, the cup lies at the front end of the centrum 

 of each vertebra to receive the condyle which 

 projects from the hinder end of the preceding 

 vertebra. Such vertebrae with the concavities in 

 front are called procoelous. In rare cases the 

 reversed condition obtains, where the condyles 

 project at the front ends of the vertebrae and fit 

 into the cups at the hinder ends of the preceding 

 vertebrae. Such vertebrae with the concavities 

 behind are called opisthocoelous. 



The vertebrae of fishes are joined together by 

 ligament, and the centra do not possess articu- 

 lar surfaces ; both in front and behind there is 

 a deep cup, whence the vertebrae are designated 

 amphicoelous or biconcave. Lepidosteus is unique 

 amongst existing fishes in having opisthocoelous 

 vertebrae. The same description of vertebrae 

 only occurs otherwise in certain Batrachia, in- 



1 R. Owen, "Anatomy of Vertebrates," London, 1866, 1. pp. 33-34. 



