IIALOSAURID^E. 295 



the species of the second group they are more opaque. It is hkely this 

 difference is due to the more advanced and perfect development of the 

 hinterns accompanied by the greatly differentiated scales on the species of 

 the second division. The type of llalosaurichthys Ale, 1889, was a species 

 belonging to the subgenus Halosaurus ; the genus was based on insufficient 

 or incorrectly interpreted characters, and has since, in 1S9G, been dropped 

 by its founder. lu their latest publications Goode and Bean and Collett 

 retain Halosaurichthys and further divide the remainder of Halosaurus into 

 two genera. Goode and Bean name their genera Halosaurus, Aldrovandia, 

 and llalosaurichthys ; and Collett names his Halosaurus, Halosauropsis, and 

 Halosaurichthys. Aldrovandia and Halosauropsis differ only in name. 

 Goode and Bean characterize Aldrovandia thus : '' Lyopomi, with ventrals 

 normal ; no second dorsal fin ; vertex scaleless ; scales of the lateral line 

 enlarged, provided with photophores. Head with pointed snout and promi- 

 nent lateral ridges. Anal fin moderate, high ; its height one third to one 

 fourth that of dorsal. Type Halosaurus rostraius Giinther." The others 

 being possessed by all the family, the only features of those mentioned that 

 serve for distinguishing characters among the species are the scales on the 

 crown and the enlarged scales on the lateral line ; but the existence of 

 intermediate species witli very thin transparent scales on the crown and 

 with scales on the lateral line moderately enlarged reduces the value of 

 these characters in a diagnosis of either genus and makes it difficult to draw 

 the line of generic separation. Collett says of his genus Halosauropsis that 

 it is " Semblable an genre Halosaunis, mais les organes lumineux de la tete 

 et de la ligne laterale sont reconverts d'une membrane en forme de sac 

 s'ouvrant en bas. Type : H. onacrochir Gthr., 1878." Probably the speci- 

 mens of Halosaurus e.xamined by this author, and with which he comjiared, 

 had been injured, as the organs of the lateral system are similar in structure 

 throughout the family ; the lanterns of //. Oivenii, and of others of the 

 group in which these organs are less developed, are provided with the very 

 thin, in cases hardly visible, membranous sac-like coverings opening down- 

 ward. In their development the enlarged scales, which bear the fusiform 

 luminous organs, have become superstructures by spreading over and cover- 

 ing the regvdar scales in their own series at each side and in the adjoining 

 series backward, thus compelling a decrease in the number of the glandular 

 bodies as the scales increased in size. The species grouped under Aldro- 

 vandia, or Halosauropsis, form the second of the subgenera mentioned above, 



