232 PALjEONTOLOOY of NKW-YOKK. 



crust presents scarcely any traces of them. When the crust is removed, however, they are 

 often very distinct ( as in PI. 61, fig. 3 g). 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 

 Gheen present no greater differences in form than maybe 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. Portlock. 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- 

 sufiicient, 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. 



Plate 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 bf c. The maxillary portions or cheeks of the buckler. These have been supplied from specimens 

 which are frequently found separated from the other parts, e b. The eyes, which are 

 * always separated with the maxillse. 



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 «, / 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. Two 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 



