518 PROCEEDINGS OF UNITED STATES NATIONAL MUSEUM. 



classes of selachians and true fishes, and the only comprehensible reason 

 why they should not be so regarded in the present case is because the 

 species are few in number. Inasmuch, however, as the function of tax- 

 onomy is to express morphological truths, and not the quantities under 

 which a given type is manifested, such reasons appear to be very insuffi- 

 cient. 

 The synonymy of the order Hyperotreta is appended. 



HYPEEOTRETA. 



Synonyms as orders. 



<:lCjclostomi, Bonaparte, Giorn. Accad. di Scienze, v, 52 (Saggio Distrib. Metod. Ani- 



mali Vertebr. a Sangue Freddo, p. 41), 1S3-2. 

 <C Helmintlaoidei, iJonoparfe, Naovi Annali delle Sci. Nat.,t. 2, p. 133, 1833; t. 4, p. 277, 



1840. 

 = Hyperotreta, Miiller, Abhandl, K. Akad. Wiss. zu Berlin, 1644, p. — , 1846. 

 = Hyperotreti, Bonaparte, Cat. Metod. dei Pesci Europei, pp.9*, 92,* 184(5. 

 <^ Cyclostomata, FUzinger, Sitzungber. K. Akad. der Wissenscb. (Wieu), B.67, 1, Abth., 



p. 57, 1873. 



BDELLOSTOMIDJE. 



Synonymy. 



= Bdellostomidse, GUI, Arrangement Families Fishes, p. 25, 1872, 

 P^tromyzontidae gen. Bonaparte, etc. 

 Myxinidse gen., Giinther, Putnam, etc. 



Hyperotreta with the branchial apertures sej^arate (C-11) and lateral, 

 debouching directly on the sides. 



POLISTOTREMA. 



Synonymy. 



= Polytrema, Girard, Report U.S.. Naval Expd. to Chili, v. 2, p. 251, 1854, (provis- 

 ional name, not of Ferussac, 1822, nor Risso, 182G.)* 

 = Polistotrema, Gill with Jordan f Gilbert, Proc. U. S. Nat. Mas., v. 4, p. 30, 1881. 

 Gastrobranchus sp., Lacvpede. 

 Heptatrema sp., Cuvier. 

 Bdellostoma sp., Miiller, Girard, Giinther, Putnam. 



Bdellostomids with an increased number of branchijie (about 10 or 11 

 on each side) and the base of the tongue between the seventh or 

 eighth pair of gills. 



* "It is to be regretted that Dunieril's appellation of Heptatrema, by referring to a 

 point of organic structure subjected to variations, couhl not be retained to designate 

 these fishes generically. If that name be restricted to the species provided with seven 

 respiratory apertures, then each species would constitute a genus by itself; that 

 with six of these aperturesoughtaccordingly be called //exa/re/na; fhen Reterotrema 

 when six are observed on one side and seven on the other ; Hepiatrema when seven ; 

 and finally Polytrema for the species described farther on. 



"Considering, however, the structui-e of the mouth, both internally and externally, 

 we would not hesitate in uniting them all under the well appropriate mrnxGoi Bdellos- 

 toma, suggested by Professor Miiller." — Girard, op. cit. 



