260 PALAEONTOLOGY OF NEW-YORK. 



Position and locality. This species is found in the lower limestone at Mineral point, 

 Wisconsin. The association of numerous other fossils known only in the Trenton limestone 

 or its representative, leave no doubt of the true position of the rock. 



{Cabinet of Mr. Conrad.) 



In addition to the preceding species, there are several others described by Prof. Green, 

 which I have not examined, except in the plaster models, and cannot therefore describe 

 them satisfactorily. These are the Asaphus tetragonocephalus, Ji. astragalotes, and A. 

 onicropleurus, with one or two other doubtful ones. I shall endeavor to give figures and 

 descriptions of these in a supplement to this work, after having examined the originals. 

 There are one or two other species heretofore erroneously referred to the Trenton limestone, 

 which arc now known to be restricted to the higher limestones. 



The strata of this period have been too little examined at distant localities, to enable us 

 to speak with certainty of the geographical distribution of the Trilobites ; but a very large 

 proportion of the species described are known at various localities within New-York, and 

 in Ohio and Kentucky in the west and southwest ; in Canada on the north, and in Wis- 

 consin on the northwest. Several of our species are evidently identical with those of the 

 lower strata of Europe, and more particularly of England, Ireland and Sweden. 



It is interesting to observe how similar are the Trilobites of our lower strata with those 

 of the same period in Ireland, given by Capt. Portlock, in his Geological Report (on 

 Londonderry, and parts of Tyrone and Fermanagh ) . It is evident that among the prevailing 

 forms, those which are identical with our species are more numerous than they are in 

 England or upon the Continent ; thus assimilating the ancient fatma of the western 

 extremity of Europe more nearly with that of the new world. Among the most numerous 

 forms of our lower strata, we may mention Isotelus and Trinucleus, which are prevailing 

 forms in Ireland. The Genus Ampyx, which is typical of the lower silurian rocks of 

 Europe, was unknown among us till obtained from the eastern prolongation of the 

 slates of the Hudson-river group in Canada, by Mr. Logan. In like manner Olenus and 

 Ogygia, which have been reckoned among the doubtful genera of our older strata, appear 

 to have existed in the eastern extension of the same, while they are quite unknown at 

 the west. Other similar changes may be noticed in tracing the geographical distribution of 

 the genera of Trilobites, both in tlie lower and upper silurian strata. 



In the present state of my knowledge on this part of the subject, the species here 

 figured, or the Trilobites of the lower silurian strata, embrace about one-third of the species 

 known below the Carboniferous period. In this remark, however, I intend to speak prin- 

 cipally of the rocks of New-York, and those portions of the adjoining Slates which I 

 have examined. It is impossible to say how far this proportion may be varied by further 

 discoveries l)oth in the older and newer strata, but it is not iiuprol)able that it may continue 

 very nearly the same ; and tliat the three great periods into which these strata are divided, 

 present almost an equal lumiljer of crustaceans. 



