TRENTON LIMESTONE. 191 



CEPHALOPODA OF THE TRENTON LIMESTONE. 



Plates XL. A. (in part), & XLI. - LVIII. 



The great abundance of a single family of this order, the Orthocerata, constitutes a 

 peculiar feature of the strata of this period. Here they attain their maximum development, 

 and at no subsequent period during the palaeozoic era do they appear in any thing like equal 

 abundance. At the base of this reck, in the Black-river limestone, I have already shown 

 (page 52) that there was a remarkal)le development of individuals of this family, though 

 comprising a few species only. During the present period, we find a large number of 

 species, many of which are exceedingly abundant and widely distributed ; while others, 

 so far as known, are of rare occurrence. 



Near the base of the Trenton limestone, where I have had good opportunities of seeing 

 the strata, the Orthoceras is very rare ; while the few species of Cyrtoceras, known in 

 the rock, seem almost confined to that position. As we ascend in the strata, specimens of 

 Orthoceras are occasionally found, but never abundantly in the lower half of the deposit. 

 The Trocholites, which differs from Lituites principally in the position of the siphuncle, 

 is abundant in the central part of the rock, occupying but a small tiiickness, and mostly 

 confined to a few localities in the vicinity of Middleville and Trenton Falls. At about the 

 same point we find the Orthocerata increasing in mmibers, and, in succeeding beds, they 

 occur in such profusion as to lie in contact with each other, imbedded in myriads. In the 

 highest beds of this limestone, in many places they have nearly disappeared, though in 

 some portions of the succeeding slate they are again abundant. It is not a little remarkable, 

 that notwithstanding the abundance of this genus of chambered shells, so few species of 

 other cephalopods siiould occur in the same strata. The two species of Lituites known in 

 the Black-river limestone, are not known in this position ; the only representative of the 

 family, known, being a single species, besides the Trocholites just noticed. 



From the immense profusion of the Orthocerata, which can scarcely be overstated, we 

 should expect to find specimens so well preserved that the entire form and structure could 

 be ascertained ; but this is not true. Almost all, and indeed every specimen found, is more 

 or less imperfect ; whether young or old, of large or small species, they have suffered 

 injury, in a greater or less degree, before being imbedded in the sediment whicii now 

 envelops them. This condition of the specimens, many of which are small, we are not 

 prepared to find, if we regard them as internal shells, where the muscular body of the 

 enclosing animal might have protected them till they were surrounded with the soft cal- 

 careous mud. In their present condition, however, we are compelled to depend upon frag- 

 ments, and to indicate the specific differences by the surface marking wiien preserved ; or, 

 in its absence, by the form of the tube, distance of the septa, and position of the siphuncle. 



In another place, I shall endeavor to illustrate the hal>its of tiic Orthocerata more in 

 detail ; showing their structure, mode of development, and other peculiarities derived from 

 the examination of a vast number of specimens obtained from tiie Trenton and Black-river 

 limestones. 



