312 



SCIENCE. 



[N. S. Vol. XVIII. No. 453. 



of the vertebrae. In a later article* written 

 mainly in review of Eepossi's paper on Mixo- 

 saurus, Yakowlew expresses the opinion that 

 the double-headed character appeared first in 

 the posterior portion of the dorsal region owing 

 to the more vigorous movement of that portion 

 of the body. It is difficult to reconcile this 

 theory with the fact that in Ichthyosaurus 

 the ribs tend to be single-headed toward the 

 base of the tail and double-headed anterior to 

 it, and that in Shastasaurus, which had also 

 a great scxilling tail, the ribs are all single- 

 headed excepting a very few immediately be- 

 hing the head. 



Concerning Yakowlew's suggestion that the 

 single-headed rib is the primitive type in this 

 order, it is certainly a significant fact that 

 it seems to occur frequently in the Triassic 

 ichthyosaurs. We should not forget, however, 

 that the oldest representative of the order 

 which has been described, viz., Quenstedt's 

 atavus, from the lower Muachelkalli, is said to 

 have a double articulation and is possibly not 

 to be referred to Mixosaurus, as has been gen- 

 erally supposed. Recent discoveries have shown 

 the existence also in the Californian Triassic 

 fauna of a form {Toretocnem,us\) /m which the 

 middle dorsal ribs are as widely forked as in 

 Ichthyosaurus. Again, it appears that in 

 Shastasaurus the double articulation in the 

 anterior part of the column is not formed by 

 reduction of the middle portions of simple 

 lateral apophyses. From the atlas to the an- 

 terior dorsal region the diapophyses are grad- 

 ually increased in height and the parapo- 

 physes reduced till the latter are mere points 

 some distance below the lower ends of the 

 diapophyses. The lateral apophyses of the 

 dorsal region, therefore, correspond to the 

 diapophyses in the cervical region. If the 

 double articulation in the neck region is sec- 

 ondary it would appear to have required the 

 addition of a lower rib head and a para- 

 pophysis. There appears, therefore, to be 

 still a chance that the double articulation is 



* ' Einige Bemerkungen ueber die triassischen 

 Ichthyosaurier.' Verhand. der Kais. Russ. 

 Mincralog. Ges., Bd. XL., Lief II. 



t See Bull. (jeol. Dept. Univ. of Calif., Vol. 3, 

 No. 12, p. 2G0. 



primitive in this group, and it may be well to 

 withhold final judgment till we have a better 

 acquaintance with the structure of the verte- 

 bral column as a whole in the later Triassic 

 forms, and particularly till we know more 

 about the Middle Triassic representatives of 

 the group. John C. Merriam. 



NOTES ON PBYSIC,^. 

 IXTERFEEKNCE OF LIGHT WITH GRE.\T PATH 



DIFFERENCE. 



The most carefully designed and con- 

 structed mechanical vibrator, such as a 

 pendulum, cannot be made to vibrate, when 

 left to itself, more than a few thousands of 

 times without greatly decreasing in ampli- 

 tude. On the other hand, the number of 

 free oscillations made by an atom of a lumi- 

 nous gas during the intervals between collisions 

 between that atom and others, during which 

 times the atom is presumably not receiving 

 energy from any source to make good its losses 

 by radiation, is, in the case of mercury vapor 

 at least, as many as 2,600,000 and that without 

 very great decrease of amplitude. This fol- 

 lows from some recent work of Lummer and 

 Oehrche, who have recently shown that a 

 given portion of the beam of light from 

 mercury vapor (the green rays) is in condi- 

 tion to interfere with a portion of the same 

 beam 125 centimeters or 2,600,000 wave- 

 lengths farther along the beam, which shows 

 that as many as 2,600,000 successive waves 

 come without a break of continuity from the 

 vibrating luminous particles in mercury 

 vapor. 



THE ELECTROMAGNETIC THEORY OF MATTER. 



The most complete and readable presenta- 

 tion hitherto made of the mathematical theory 

 of the motion of minute electric charges 

 (electrons) is given by Max Abraham in 

 Drilde's Annalen for January, 1903. It is 

 by comparison of the results of this mathemat- 

 ical theory with results of experiments on 

 cathode and Becquerel rays, on the Zeeman 

 effects, etc., that the electromagnetic theory 

 of matter has arisen. 



An electrically charged body in motion has 

 more momentvun and stores more energy, in 



( 



