TRENTON LIMESTONE. 
203 
annulations, and their more extreme undulation upon the back, as well as the different 
direction of the strise and position of the siphuncle. 
Specimens of this species, as well as of the others, are often much flattened, presenting 
an elliptical section ; and the specimens which I have, show all grades in this respect, from 
circular to extremely elliptical, proving the latter forms to be due to compression; which 
is probably true of all, or nearly all, the elliptical species of Ortiioceras. 
Fig. 7 a. Ventral side of a fragment, showing the nearly transverse direction of the annulations. The 
strias are scarcely preserved. 
Fig. 7 b. Dorsal side of the same; the specimen cylindrical. 
Fig. 7 c. Transverse section of the same. 
Fig. 7 d, e. Two smaller fragments of the species, compressed in different degrees. 
Fig. 7 f g. Transverse sections of the two last. 
Fig. 7 h. An enlarged portion of the surface. 
Fig. 7 i , k. Another fragment and section, showing the position of the siphuncle. 
The Orthoceratites trochlearis of Hi singer is from the more ancient calcareous strata of 
the palaeozoic period, in Dalecarlia (Sweden), corresponding - in position to our Trenton 
limestone ; which circumstance, together with the similarity in form and markings, induces 
me to make a comparison between the two, believing at first that they were identical. 
Position and locality. In the lower shaly layers at Middleville. 
267. 20. ORTHOCERAS ( Species undetermined ). 
Pl. XLIII. Fig. 8. 
Cylindrical, very gradually tapering; annulations rounded, undulating, distant a little 
more than one third the diameter ; section circular ; septa moderately convex ; siphuncle 
central. 
This fragment possesses some peculiarities, which indicate a difference between it and 
those already described ; the annulations are more rounded than those of any other species, 
except the 0 . textile and 0 . bilineatum. 
Position and locality. At Middleville, in the lower shaly strata of the Trenton limestone. 
I have discovered several other imperfect specimens of annulated Orthocerata in the 
shaly strata of the Trenton limestone ; but the surface markings are either obscure or 
obliterated, so that it is scarcely possible to point out specific distinctions. Nearly all these 
specimens are more or less compressed, so that the sections present variable elliptical forms, 
and this compression distorts the annulations in such a manner that these cannot be relied 
upon for specific distinction. In the absence of surface markings, therefore, it is impossible 
to characterize species with any degree of certainty. 
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