ZOOLOGY. 789 



fauuas show no greater resemblances tliau the siuiilarity of physical con- 

 ditions on the two sides would lead us to expect. 



These conclusions contrast with those of Dr. Giinther. "Dr. Giinther 

 assumes that nearly one-third of the total number of species of marine 

 fishes on the two shores of tropical America will be found to be iden- 

 tical. Ucnce he infers that there must have been, at a comparatively 

 recent date, a depression of the isthmus, producing an intermingling 

 of the two faunas." 



Professor Jordan explains that "this discrepancy arises from the 

 comparatively limited representation of the two faunse at the disi)o.sal 

 of Dr. Giinther. He enumerates 193 marine or brackish water species, 

 as found on the two coasts, 59 of which are regarded by him as speci- 

 fically identical, this being 31 per cent, of the whole. But in 30 of these 

 59 cases" Professor Jordan regards " the assumption of complete identity 

 as erroneous, so that, taking the number 193, as given," he would "re- 

 duce the percentage to 15. But these 193 species form but a fragment 

 of the total fauna, and any conclusions based on such narrow data are 

 certain to be misleading." 



Some data respecting the supposed con-specific forms are interesting. 

 " Of the 71 identical species admitted in our list, several (e. g., Mola, Or- 

 cyniis) are pelagic fishes, common to most warm seas ; still others {e. (/., 

 TracMcriiH, Caranx, Diodon sp.) are almost cosmopolitan in the tropical 

 waters ; most of the others (e. g., Gohins, Gerres, Gentropomus, Galeich- 

 thys sp., &c.) often ascend the rivers of the tropics, and we may account 

 for their dilfusion, perhaps, as we account for the dispersion of fresh- 

 water fishes on the isthhius, on the supposition that they may have 

 crossed from marsh to marsh at some time in the rainy season." But 

 " in very few cases are representatives of any species from opposite 

 sides of the isthmus exactly alike in all resi)ects. These differences in 

 some cases seem worthy of specific value, giving ' us representative spe- 

 cies,' on the two sides. In other cases, the distiuctious are very trivial, 

 but in most cases they are appreciable, especially on fresh specimens." 



Finally, Professor Jordan is " brought to the conclusion that the fish 

 faunsB of the two shores of Central America are substantially distinct, 

 so far as species are concerned, and that the resemblance between them 

 is not so great as to necessitate the hypothesis of the recent existence 

 of a channel across the isthmus, i>ermitting the fishes to pass from one 

 side to the other." 



These results are especially noteworthy inasmuch as they confirm 

 and are themselves corroborated by the results of studies of various 

 other classes of the animal kingdom. [Proc. U. JS. Nat. 3Ius., v. 8, pp. 

 361-394.) 



Selachians. 



Glassification of the Sharks. — An important contribution to our knowl- 

 edge of the skeletons of sharks, and some excellent hints as to the 

 classification of the group, have ap[>eared in an article by Prof. William 



