218 W. Crookes—Lines of Molecular Pressure. 
under the name of Meekoceras, and refers the others, doubtfully, 
because of the imperfection of the specimens, to Arcestes Suess. 
ow comes an interesting and, in view of the recognized St. 
Cassian age of the Nevada Trias before referred to, a somewhat 
unexpected fact. The types of these Cephalopods, as dis- 
tinctly stated by Professor Hyatt, have close affinities with 
those of the Muschelkalk or Middle Trias of Europe, and not 
with those of the St. Cassian, Aussee, and Hallstadt deposits of 
the European Upper Trias. This fact, however, need not 
from that of the Nevada Trias; and, furthermore, that the 
positions which they occupy are respectively adjacent to east 
and west borders of an extensive area which was above the 
the first mentioned strata, although now known only in a lim- 
ited area, and of the distinctively Triassic facies of its types, 
especially the Cephalopods, it seems not improbable that we 
may yet find a physical plane for their separation as Jurassic 
and Triassic groups respectively. 
Art. XXVI—On the Illumination of Lines of Molecular Pres- 
sure, and the Trajectory of Molecules ; by WILLIAM CROOKES, ~ 
3, V.P.C.8.* 
Induction Spark through Rarefied Gases.—Dark Space round 
the Negative Pole——The author has examined the dark space 
which appears round the negative pole of an ordinary vacuum: 
tube when the spark from an induction-coil is passed through 
it. He describes many experiments with different kinds of 
poles, a varying intensity of spark, and different gases, 
arrives at the following propositions :— 
* Abstract of a paper read before the Royal Society, Dec. 5, 1878. (Phil. Mag» 
Jan., 1879.) : 
