ZOOLOGY. 501 



various systems. (Comptes Rendus Acad. Sc. Paris, cii, pp. 1573-1575 j 

 J. E. M. S. (2), VI, pp. 777, 778.) 



VERTEBRATES. 



FAslies and fish- like forms. 



Factors in the geograpliical distrihution of American fresh-icater fishes. — 

 Professors Jordan and Gilbert have published a " List of fishes collected 

 in Arkansas, Indian Territory, and Texas," and have concluded with 

 some general considerations on the distribution of the species : 



(1) Many species have been regarded as very local, but research has 

 shown that some of them have " a very wide distribution in the West 

 and South, and what is true of these species will very likely be found 

 true of all these now known from only a few localities ; " such is espec- 

 ially the case with the small percoid fishes named Etheostomin^e or 

 Ethiostomatin?e. 



(2) " As our knowledge of the geographical range of a species widens, 

 it becomes necessary to extend our ideas of the range of variation in- 

 cluded by it, and we are compelled to admit under it geographical vari- 

 eties or subspecies." Eventually, therefore, a trinomial nomenclature 

 must be adopted in ichthyology, as has been done in ornithology. 



(3) "The environment and conditions of life being similar, the water 

 communication being free, we have a similar fauna in regions widely 

 separated." For example, the fishes of the Ozark region are substan- 

 tially identical with those of the hilly regions of Tennessee. 



(4) "Free water communication is essential to a varied fauna," and 

 "the larger a river system the greater" is the "number of species in 

 each of its affluents," while, on the contrary, short streams emptying 

 into the ocean have a comparatively meager fauna. 



The factors "favorable to the production in any stream of a large num- 

 ber of fishes" are (1) "clear water;" (2) "a moderate current*," (3) "a 

 bottom of gravel, preferably covered by a growth of weeds;" (4) " water 

 not too cold and not stagnant;" (5) connection with a large hydro- 

 graphic basin ; and (6) " little fluctuation in the year in volume of the 

 stream or in the character of the water." 



The conditions enumerated, it is added, are " well realized in the 

 Washita Eiver and in certain affluents of the Ohio and the Tennessee, 

 and in these, among American streams, the greatest number of species 

 has been recorded." (Proc. U. S. Nat. Mus., 1886, pp. 1-25.) 



Selachians. 



Relations of the extinct Hyhodonts. — The Hybodonts were sharks 

 numerous in the ancient seas, and which have generally been supposed 

 to be closely related to the Cestracionts or Heterodontids, although 

 much doubt has existed in the minds of some ichthyologists as to their 

 exact affinities. Some remains of the lower jaw and the hyoid arches 



