THE AMERICAN TETRAGONOPTERINAE. 229 



also taken on forms considered specifically distinct; this species admirably 

 shows the divergence of species with isolation in the rivers independently empty- 

 ing into the Atlantic in eastern and southeastern Brazil and in Central America 

 and Mexico. In the latter area it has given rise to a series of species or varieties, 

 A. mexicanus, A. viacrophthahnus, and A. nicaraguensis, distinguishable from 

 each other but scarcely distinguishable from the mdependently originated varie- 

 ties and species of southeastern Brazil. 



In many cases the varieties are statistical, i. e., while specimens of the two 

 vai'ieties cannot be disting-uished, a large number of specimens from one locality 

 are in the aggregate different from a large mmiber of other specimens from 

 another locaUty. The details are given under the respective species. 



The genus shades in many directions into genera here treated as distinct. 



There is considerable variation in the size of the mouth, from the small 

 mouthed A. alleni, A. pellegrini, A. festae, etc. to A. fasciaius, A. scabriceps, 

 and others that lead to the genus Astyanacinus. Similarly there is an irregular 

 variation in the number of teeth in the maxillary from none to three or four 

 or even ten. Of special interest is the variety A. nicaraguensis, in which the 

 dentition of the maxillary shows a complete gradation fi'om typical Astyanax 

 to the species segregated as Hemibrycon. The caudal sheath is always small 

 but here also in A. taeniatus intermedius there is an indication of a tendency 

 toward Moenkhausia. In all the specimens of the species of Astyanax examined 

 except A. mutator the lateral line was complete; in specimens of Hemigram- 

 mus inconstans the lateral line is complete in some and in others it is incomplete. 

 On the other hand, of the hundreds of specimens of many other species of Hemi- 

 grammus, but one or two were found with a complete lateral line. The details 

 are given under the respective species. In A. festae, A. regani, A. albeolus, 

 some specimens of A. metae, and half the specimens of A. rivuJaris from the Rio 

 San Francisco there were but four teeth in the inner series of the premaxillary, 

 one of the characters distinguishing Bryconamericus. In A. goyacensis, A. 

 niultidens, A. gracilior, and A. paucidens, we have the entire cheek covered 

 with the second suborbital, one of the other characters distinguishing Brycona- 

 mericus. In A. taeniatus the teeth of the sides of the lower jaw tend to become 

 graduate, a condition leading to the distinguishing character of Deuterodon. 

 In A. alleni the scales are ciliated and lead toward Ctenobrycon. The genus 

 is thus seen to branch out in various directions, the ends of the branches being 

 here considered as distinct genera. 



