Janitabt 29, 1909] 



SCIENCB 



193 



until identical illumination lias been obtained, 

 this diversity of photographic action, due to 

 variation of exposure, or of development (of 

 course with the proviso that such variations 

 are never excessive), is apparently taken care 

 of. At any rate, I find no difference in the 

 results vyhich can be traced to this cause. 



Admitting that the little a band is four and 

 one half times as intense in the spectrum of 

 Mars, if we may assume that the intensity of 

 the band is proportional to the total amount 

 of vapor present in the combined air columns 

 traversed by the rays, as it is very nearly in 

 the case of incipient absorption, it is perhaps 

 permissible to say, since the rays pass twice 

 through the atmosphere of Mars, that on 

 the average Mars has 0.5 X (4-8 — 1.0) = 1.Y5 

 times as much aqueous vapor in its atmos- 

 phere as that which exists above Flagstaff in 

 the month of January, or roughly, since one 

 and three quarters times the amount of water 

 vapor in the surface air of Flagstaff would be 

 2.1Y grains per cubic foot, or 5.0 grams per 

 cubic meter, it may be concluded that the 

 dew-point on Mars would be 33° F., if the 

 distribution of moisture were the same in the 

 upper air of the two planets. In this respect, 

 however, there is a very wide divergence of 

 conditions on the two worlds, since, as I have 

 shown in my paper on " The Greenhouse 

 Theory and Planetary Temperatures," in the 

 PhilosopMcal Magazine for September, 1908 

 (p. 469), the proportion of aqueous vapor 

 existing at great elevations above the surface 

 on Mars is very much greater than here. 

 This is due to the comparatively rare atmos- 

 phere of Mars, to the low boiling point of 

 water on that planet where water evaporates 

 much rtiore readily than here, and to the pre- 

 vailing desert conditions, that is to say, to 

 the infrequeney of those atmospheric condi- 

 tions which conduce to the formation of cloud 

 and rain. Through these causes, aqueous 

 vapor on Mars diffuses to greater heights and 

 remains suspended in the air for longer inter- 

 vals than with us. As a consequence, al- 

 though there may be a very extensive pro- 

 tecting mantle of highly absorbent vapor 

 which prevents surface radiation and con- 

 serves surface, temperature, the dew-point 



at the surface remains low, probably seldom 

 rising much above the freezing point, and 

 the prevalent conditions on Mars are those 

 of a mild but desert climate, as Professor 

 Lowell has all along maintained. 



Frank "W. Vert 

 Westwood Astbophysicai, Obsebvatort, 

 West WOOD, Mass. 



THE SELACHIANS ADMITTED AS A DISTINCT CLASS 



Ever since 1873 (Am. Journ. Sci. (3), 6, 

 434, 435) I have claimed class rank for 

 the Selachians or Elasmobranchiates. This 

 view has been later accepted by most Ameri- 

 can ichthyologists, and notably by Jordan, 

 since 1902.' At last two European natural- 

 ists, of great eminence, have come to the 

 same conclusion. 



Professor A. A. W. Hubreeht, in the Qtiar- 

 terly Journal of Microscopical Science for 

 November, 1908 (p. 156), has stated that " a 

 division of the vertebrates in the superclasses 

 of Cyclostomata, Chondrophora, and Osteo- 

 phora might suggest itself, Amphioxus re- 

 maining yet more isolated in its superclass 

 of Cephalochordata. The Chondrophora 

 would then contain the Elasmobranchs, the 

 Osteophora all the other higher vertebrates." 



Mr. C. Tate Regan, in the Annals and Mag- 

 azine of Natural History for January, 1909 

 (8. ser., 3, 75), has recalled that he had "al- 

 ready expressed the opinion that the true 

 Fishes are at least as distinct from the Sela- 

 chians, on the one hand, and the Batrachians, 

 on the other, as any of the vertebrate classes 

 are from each other, and are equally entitled 

 to rank as a class,"' and insists on their claim 

 to class distinction. 



Hubreeht and Regan, it is true, are not the 

 first or only European naturalists to differ- 

 entiate the Selachians as a class from the 

 Pisces, for Geoffrey Saint-Hilaire and La- 



'Up to 1887 Jordan had regarded the Selachians 

 as a "class Elasmobranchii " ; from 1888 to 1902 

 he associated them with the Pisces under two sub- 

 classes, Selachii and Holocephali; in 1902 he 

 reverted to his former view. ( See " Guide to the 

 Study of Fishes," I., 1905, pp. 506, etc. 



^^ Regan, Proc. Zool. Soc, 1906, p. 724, and 

 "Biol. Centr.-Am.," Pisces, p. viii (1908). 



