October 2, 1890] 



NATURE 



551 



in other words, the bright fluting of carbon in the green 

 and blue of the subsidiary swarm will just mask the 

 absorption bands. They will pale, and the colour of the 

 star (red) will be but slightly affected from this cause ; but 

 the blue flutings will be clear gain to the blue end of the 

 spectrum, and crimson will result. 



If this explanation be conceded, it is clear that comets 

 travelling round such stars are conditioned very much 

 like comets travelling round our own sun. 



The general colour of the stars in Group VI. indicates 

 that they are near the point of invisibility, the conditions 

 being no doubt a red or white hot crust with a strongly 

 absorbing atmosphere. It is worth while to point out 

 that the cessation of all radiation of light from the central 

 body need not prevent its passing on as a variable star to 

 Group VII. As we must assume comets to be shreds of 

 nebula;, i.e. meteoritic fields or streams, filched by masses 

 which pass near them ; and as the mass remains after the 

 light has gone, there will be the same attraction at work, 

 and we have no right to assume that it will not act in the 

 same way as heretofore. 



We can gather from this that practically there can be 

 no permanently dark bodies in space ; they imist at one 

 time or another be accompanied by comets, and they 

 must therefore be variables. 



Here a most interesting point comes in : if the pheno- 

 mena of the repulsion of comets' tails, or, in other words, 

 the repulsion of carbon in some form or other from 

 cometary swarms, depends upon the thermal energy of 

 the central body, this result can no longer happen when 

 the central body has cooled down. The effect of this 

 upon the spectrum of such a compound system is well 

 worth inquiring into. 



In the hypothetical curves I have already given, I have 

 dealt with simple cases. But in the stars there will be 

 certain to be complex ones brought about by the succes- 

 sive periastra or apastra not being coincident in the two 

 swarms (to deal only with two), and by different relation- 

 ships in the periods. 



I append (Figs. 7-10) some hypothetical curves worked 

 out both in light-units and magnitudes, the conditions 

 being stated for each. The paucity of actual light- 

 curves available prevents any inquiry as to the stars in 

 which the conditions here imagined actually exist, but in 

 the absence of such knowledge it is still easy to gather 

 that different periods separating maxima, secondary 

 minima of unequal periods, and great variations in the 

 rise to and fall from maximum, instead of necessarily being 

 the result of "irregularity," are all demanded by the 

 most perfect regularity, provided we have more than one 

 swarm to deal with under conditions anything like those 

 employed in the hypothetical curves above given. 



If there is anything of value in what I have advanced, 

 it is quite clear that the observations of variable stars and 

 variable star catalogues require considerable revision. 

 First, arrangements should be made with the obser- 

 vatories of America and India so that the observations of 

 a certain number of stars in the northern hemisphere 

 should be observed as continuously as possible. The 

 relative brightening of the bright carbon flutings in stars 

 of Groups I. and II., and the paling of the dark carbon 

 flutings in Group VI., should be spectroscopically 

 watched in each case. 



It is highly important also that the precise group to 

 which each variable belongs should be determined at 

 once, and that this datum should take the first place in the 

 working catalogues employed. 



The observations should also be recorded when made on 

 light-curves, the time ordinate being contracted as much as 

 possible in order that the genesis of the compound curve 

 may be suggested as soon as possible, so that future ob- 

 servations can be controlled, and the greatest attention be 

 given at the critical periods. 



The colour-observations have done their work and have 

 had their day : less attention need now be directed to 

 them, and much time will be liberated thereby.^ 



J. Norman Lockyer. 



NO. 1092, VOL, 42] 



THE LABYRINTHODONTS OF SIVABIA,^ 



SWAB I A, it need scarcely be said, lies to the south- 

 east of Stuttgart— the classic ground of the Triassic 

 Labyrinthodonts of Germany — and since it contains the 

 same Triassic deposits, we should naturally expect to 

 find therein the same species of this group of Amphi- 

 bians. The present memoir, remarkable alike for the 

 splendid plates with which it is illustrated, and for the 

 care with which the specimens have been described, is 

 devoted to making known to the scientific world the 

 magnificent collection of Labyrinthodont remains which 

 have been from time to time obtained from the Swabian 

 and WUrtemberg deposits, and are now preserved in the 

 Museums at Stuttgart, Tubingen, and Munich. Of the 

 seventeen plates with which this work is illustrated, a 

 large proportion are folded ones of very considerable 

 size, while all are especially noticeable for their beauty of 

 execution. They appear to have been printed by some 

 special process from photographs, the finely-preserved 

 specimens of skulls standing out with wonderful clearness 

 from a black background. Even more noteworthy than 

 the unrivalled execution of the plates is the perfection 

 and beauty of the specimens themselves ; and we would 

 especially direct attention to the magnificent skull of 

 Metopias, represented in plates xii. and xiii. of the work 

 before us, as being the finest Labyrinthodont specimen 

 that has ever come under our notice. 



In the introduction to his memoir, the author, after 

 summarizing what has been previously written on the 

 subject, glances at the chief groups into which it has 

 been proposed to divide the Labyrinthodonts. The 

 forms treated of in the present memoir all belong to 

 the typical group, the Euglypta of the British Association 

 Committee of 1874, and the Stereospondyli of Prof, von 

 Zittel. This group has been generally characterized, 

 among other features, by the fully ossified centra of the 

 vertebraj ; but Dr. Fraas remarks that it is very difficult 

 to be sure of the nature of their vertebrae, and that, at 

 least m. Mnstodonsaurus, either the caudal vertebra;, or the 

 vertebrae of young individuals, are of that imperfectly 

 ossified and segmented type to which the term " rhachi- 

 tomous " has been applied, so that fully ossified vertebrae 

 only occur in the trunk region of the adult. This seems 

 to us to indicate very clearly that the pre-Triassic Arche- 

 gosatirida, in which the vertebra; are always " rhachi- 

 tomous," can only be separated from the Anthracosaurido! 

 and MastodonsauridcB by characters of family value. 



After the introductory portion, the author proceeds to 

 discuss the geological divisions of the German Trias. 

 Here, contrary to the views hitherto generally adopted, 

 four main stages or groups (exclusive of the Rha}tic) are 

 recognized, viz. the Bunter-sandstein, the Muschelkalk, 

 the Lettenkohle, and the Keuper. The separation of 

 the Lettenkohle as a primary group distinct from the 

 Keuper (in which it has hitherto been generally included) 

 appears to rest on the ground that it contains many forms of 

 Vertebrates common to the underlying Muschelkalk which 

 do not occur in the typical Keuper. Thus the Sauro- 

 pterygian genera Nothosaurus and Simosaurus range up 

 into the Lettenkohle, but stop short of the true Keuper. 



• I have to thank the Astronomer-Royal and Prof. A. S. Hcrschel for the 

 correction of an error into which I had fallen in the part relating to light- 

 units in the first draft of this article. The table on p. 546 I have extracted 

 from one of the valuable letters with which Prof. Herschel has favoured me 

 on this subject ; it is fuller than the one it repLaces. „._.,.„ 



" " Die Labyrinthodonten der schwiibischen Trias, by Eberhard traas. 

 PalaoHtographka, vol. xxxvi. (1889), pp. 1-158, plates i.-xvii. 



