observed, are even more variable than the proportions of length 
and breadth, and neither can be relied upon in the absence of 
other characters. The characteristics of Triarthrus as given by 
Prof. Haldeman are doubtless the faithful portrait of an individ- 
ual, but do not characterize the species. As well might we ex- 
pect a cylinder to present the same form and proportions to the 
eye as it becomes flattened or spread out, as that any body hav- 
ing the form and characters of a trilobite, should retain them un- 
der all conditions and circumstances of compression and lamina- 
tion which have befallen the strata in which it is imbedded. 
In one of the specimens of the so-called Atops, and in the one 
which Prof. H. examined, the body is partially folded and the 
front of the buckler bent downwards, giving it a less apparent 
proportional length, and from the same cause the middle lobe is 
more convex, while subsequent pressure has extended the whole 
in its lateral dimensions. In another specimen of this fossil, the 
_ proportions of the parts of the buckler vary materially from those 
given in the description.* The body is said to be distinguished 
by having the lobes equal in width, while the specimens figured 
(as just cited) show an equal variation with those of the 'Triar- 
thrus from the Utica slate. wi 
There is another very distinctive character in this fossil which 
appears to have escaped the observations of, or if observed is not 
mentioned by, Dr. Emmons or Prof. Haldeman, and this is, that 
the middle lobe of the thorax is marked by a row of small spines 
or tubercles which are distinct in the specimens, but more par- 
ticularly in the moulds or impressions made by the crust which 
is nearly or quite destroyed. These spines have exactly the 
character of those in the Triarthrus, while the papillose or granu- 
lated surface in the two fossils is precisely similar. It should be 
been applied, are found in shales which are not only thinly lam- 
inated but extremely contorted or folded and partially altered in 
lithological character, so that we should expect distorted forms 
and proportions in fossils since we know that this always happens 
under such circumstances. 
- y aspect 
( Triarthrus) Calymene i—Cephalic shield semi-elliptical or 
Nae ee ee ae ED ee ee ete he eee eee 
