438 ‘Canadian Record of Science. 
Under the division of Magnidorsati are the following divisions : 
1. Aequali. The ventral side rounded without a sharp keel in 
the middle.—Vaginatus to Lituite limestone. 
2. Carinati. The ventral side sharply keeled, with the lip and 
~ lines of growth of the middle field, forming either a single strongly 
projecting obtuse angle, or. two side arches, separated by an in- 
ward curvature The channel bordered outward by a single 
strongly developed longitudinal raised jine.-—Olenus Zone to Vagi- 
natus limestone. 
These are the main divisions of Dr. Holm’s classification, but 
each is sub-divided into one or more sections and sub-sections, and 
the Swedish type species of each section mentioned, so that the 
scheme forms a complete key to the classification of the Swedish 
species, and a most valuable reference for the Hyolithoid form of 
all countries. ; ‘ 
This classification, based as it is on so large an amount of 
material, will be of great service in the future study of this group of 
fossils. As it is largely based on external ornamentation it is 
more readily available than if it turned entirely on internal struc- 
ture. It has accomplished for Hyolithes what de Verneuil’s classi- 
cal work did for Orthis nearly fifty years ago, but in a much more 
complete and systematic way. By treating the genus in its rela- 
tions to time and space it brings out the genetic relations of the 
different sections of Hyolithes, and shows the simpler forms to have 
been the earlier. 
The tendency of opinion in modern times, however, is to the 
breaking up of large and unwieldy groups such as. Hyolithes and 
Orthis, and so we think that future writers will, by using lines of, 
descent, endeavour to perform this service for Hyolithes; as it has 
been done recently for Orthis by Messrs. Hall and Clarke. What-: 
ever may be attempted in this way in future years it is. certain 
that Holm’s classification of Hyolithus will be found exceedingly. 
useful. ; ‘ hk 
There are other features in. this work well worthy of study. 
Following the example of.Barrande in his work on the Pteropoda> 
of Bohemia. * Dr.. Holm .devotes considerable. space to the geo- 
graphical distribution and vertical range af the different species of 
Hyolithes. Systematic ahd complete tables are given of the occur- 
rence of species in Sweden, Norway, Denmark, Russia, Great Bri-, 
tain, Bohemia, Canada, United States and other countries. . Also a 
1 Russia and the Ural Mtns, 1845, 
2 Systéme Silurien de la Bohéme; Vol; ILI- : ; Pees, 
3 Genera of Paleozoic Brachiopoda, Part I, 1892. 
