232 PALEONTOLOGY OF NEW+YORK. 
crust presents scarcely any traces of them. When the crust is removed, however, they are 
often very distinct (as in Pl. 61, fig. 3g). When the thick crust of the margin of the 
caudal shield is separated, the lower part presents a striated surface, which is also common 
to corresponding parts in the buckler, and apparently to the entire under surface of the 
animal, as well as to those portions of the articulations which come in contact with each 
other. 
After examining several hundred specimens of all sizes, and in every possible degree of 
perfection, I am satisfied to refer all those of New-York to a single species. The models of 
GREEN present no greater differences in form than may be witnessed in specimens ordinarily 
found, and which, if regarded as specific, would give us a different species for nearly every 
individual ; depending on difference of size, in the manner of being imbedded, the nature 
of the mass in which it is found, etc. The same remark will apply to several of the species 
given by Capt. Porriock. I have therefore thought it better to bring back the whole to the 
original species ; believing that such a course will cause far less difficulty and confusion, 
than an attempt to identify and establish distinct species upon characters which are in- 
sufficient, or so variable as not to be relied upon. 
Notwithstanding the great number of individuals which existed at the period of the 
Trenton limestone, it is extremely rare to meet with perfect specimens. This is, doubtless, 
in part owing to their great size, which exposed them to be more frequently broken, than 
the smaller species; but it would appear, also, that they were more easily separated at 
their joints and sutures, than some other species, as shown from the condition of the parts 
which we find. Owing to this circumstance, we are able to illustrate its structure more 
perfectly than in most other species; and I am induced to present many of these figures, not 
only with this view, but also to aid students in difficulties which I find are not uncommon 
in identifying isolated portions of the fossil. 
Pirate LX. 
Fig. 7 a. A specimen preserving the thorax, caudal extremity, and central lobe of the buckler. (The 
base of the latter is too far extended in the engraving.) The caudal shield is denuded of the 
shell. 
Fig. 7 6, c. The maxillary portions or cheeks of the buckler. These have been supplied from specimens 
which are frequently found separated from the other parts. EE. The eyes, which are 
always separated with the maxille. 
Fig. 7 d. The inside of the lower crust of the cephalic shield, the upper part having been separated at 
the lateral suture. 
Fig. 7 e, f. The lower side of the cephalic shield, towards the extremities of which the upper and lower 
crusts are folded together, so as to present the appearance of projecting spines. 
Fig. 7 g. The epistoma or labrum, the place of which is indicated in the centre of the lower side of the 
previous figure. The epistoma is rarely found connected with the head of the fossil. It was 
attached by a suture, and probably easily separated. T'wo circular depressed spots are re- 
presented in the figure, which are always observed in well preserved specimens : these 
probably indicate the points for the attachment of muscles or tendons upon the inside. The 
