56 PALEONTOLOGY OF NEW-YORK. 
The rock is very compact, and the fossil cannot be easily obtained except in weathered 
portions of the mass. Most of those figured, and nearly all the specimens usually seen, 
are found upon the weathered surfaces of the rock, where it presents such a variety of 
appearances as to lead one to believe there are several species. In examples like those on 
Plate XVI. they are commonly regarded by the inhabitants of the neighborhood as the 
vertebre of fishes, to which they bear some remote resemblance. 
T have little doubt of the identity of this fossil with those cited above ; though since there 
is not absolute certainty, I have adopted another name until this can be determined. The 
specimens of Mr. Sroxss are from a locality where the existence of other well known fossils 
induces us to refer the rock to the lower stages of our system, and probably to a position 
precisely corresponding to the thin mass containing these fossils upon the Black river. The 
specimens of Mr. Troost, though said by that gentleman to be associated with many from 
the younger silurian strata and some carboniferous fossils, [ am well aware are from a 
position quite as low in the series as the Trenton limestone of New-York. It is true, Mr. 
Hisrtneer has referred his Orthoceratites crassiventris to the more recent transition strata of 
Sweden, which would be a strong indication of its being a different species, if there be no 
error in regard to position. 
Puate XV. 
Fig. 1. A fragment of this species, partially covered with the delicate thin outer shell, presenting in the 
lower part of the figure some obscure markings of the septa; and above, showing the 
moderate convexity of a septum. 
Fig. 1 a. Transverse section of the same specimen, broken through the annulations of the siphon. 
Fig. 1 4, A portion of the outer shell magnified, showing the fine undulating threadlike lines upon the 
surface. (Specimen from the Cabinet of the Albany Institute.) 
Fig. 1 ¢. A longitudinal section of a portion of a large individual, showing the septa, which are com- 
posed of double plates reaching only to the inner lamina of the exterior shell. The distance 
between the lamin is unusually great in the upper part of this specimen, the spaces between 
them being nearly as great as between the septa. 
Piuate XVL. 
Fig. 1. A transverse section, showing the edge of an annulation of the siphon, and the proportionate size 
of this part of the fossil. (By a mistake of the artist, the upper edge of this ring is far too 
nearly central to be true.) 
Fig. 1 a. An artificial longitudinal section of a portion of fig. 1, pl. 15, showing the siphon and septa. 
Fig. | b. A longitudinal section, in a direction from the ventral to the dorsal side, showing very clearly 
the position of the siphuncle. The specimen appears contracted at both extremities, which is 
due to its having been slightly bent, so that in wearing down, the two extremities are worn 
beyond the centre, while the middle portion is central, showing the siphuncle almost in 
*The structure of siphuncle here represented, though common in the lower, is rare in the higher strata; and 
although we have something approaching to the same form, yet I have not hitherto been able to detect precisely the 
same in any of the Orthocerata of more recent periods than the Hudson-river group. 
