188 REVIEWS—CONTRIBUTIONS TO PALHZONTOLOGY. 
names. In the work before us, Professor Hall proposes two new 
genera: Reteograptus and Thamnograptus. The former is charac- 
terised by the reticulated structure of the entire stipe (as in the G. 
tentaculatus of Point Levy, described by the author in the Geological 
Report of the Canadian Survey for 1857) and thus resembling to 
some extent the genus Retiolites or Gladiolites of Barrande, but the 
serratures do not reach the central cells. In the new genus Thamno- 
graptus, on the other hand, the stipe appears to be entirely destitute 
of cells or serratures of any kind. 
Amongst the Brachiopods, several new genera, in addition to those 
described more or less recently by the author, are also proposed; but 
the data on which these are founded, appear to be somewhat unsatis- 
factory. As regards fossil forms, which so greatly outnumber living 
species in this class of mollusca, the classification-characters, indeed, 
are beset in their application, and throughout the entire group, with 
almost insurmountable difficulties. The earlier genera were estab- 
lished, to a great extent, on external characters of more or less easy 
employment; but it soon became evident that many species were thus 
placed in forced or artificial collocation: as, although alike exter- 
nally, their inner structure was frequently found to be entirely dis- 
tinct. Thus, the forms of the genus dthyris became separated from 
Terebratula (or mostly so,) by their internal caleareous spires, and 
placed properly amongst the Spiriferide, although a straight hinge-line 
was originally thought to be one of the essential characters of these latter. 
In Terebratella, again, the supposed arched hinge-line of the Tere- 
bratule was shewn to be an uncertain or artificial character. But many 
internal points of brachiopodous structure, besides being of difficult, 
and frequently of impossible observation, may be also to some extent 
of little value as natural classification-elements. It seems clear, at least, 
that subdivisions based on minute and subordinate internal characters, 
may be pushed too far. All paleeontologists must agree that it is at 
present next to impossible to refer certain fossils of this class to their 
proper genera, and the difficulty will not only be much increased by 
minute generic distinctions founded on characters that cannot be 
observed in the great majority of examples, but each separate species 
bids fair to become eventually the type of a distinct genus. 
The new genera proposed by Professor Hall are named respectively, 
Skenidium, Ambocelia, Vitulina, Meristella, and Leiorhynchus ; with 
Rhynchotrema as a doubtful genus, founded on the old Rhynconella in- 
