XX INTRODUCTION. 
of England. The Leptena sericea, L. tenuistriata, and probably L. alternata, are identical 
with Lower Silurian species in England, being well known and common forms both here 
and in Europe. The Delthyris lynx seems equally common in the lower strata of the United 
States and Russia; while Orthis testudinaria is every where characteristic of strata of this 
age. Other species of Brachiopods are closely allied, and may be regarded as representatives 
of European species. In this class, the Leptena and Orthis are far more conspicuous and 
important than the érypa, the species of which are fewer and generally less widely dis- 
tributed. Of the latter genus, however, two species, thé 2. modesta and A. increbescens, are 
abundant and widely distributed, being every where typical of the lower division of the 
system. 
So few species of AcrpHata from this period have been described in Europe, that no 
very satisfactory comparison can be drawn between them and our own. We recognize, 
nevertheless, several forms very analogous to those of New-York and the Western States. 
In this class I have established several new genera, among which are the Modiolopsis, 
Ambonychia and Tellinomya : the two former of these are well characterized in several 
species, while the latter now embraces some that are probably heterogeneous forms.* 
These may be regarded as paleozoic genera, and the species thus far indicated are im- 
portant in distinguishing the older strata. The number of species of this class is far greater 
than could have been anticipated, being more than half as many as of the Brachiopoda, 
and are therefore entitled to our consideration. 
Among the Gasrropops, there is a very close resemblance in several species ; while 
among those which appear to be identical with European forms, we are able to assert 
positively only one or two of which no question remains.t In this class, we have several 
genera peculiar to Lower Silurian strata. Among these may be included Ophileta, Raphi- 
stoma, Scalites, Maclurea, Bucarnia and Cyrtolites. The species in this class, although 
numbering several genera, are not numerous till we arrive at the base of the Trenton 
limestone. The generic form Bucania, which I have separated from forms heretofore re- 
ferred to Bellerophon, we now know to belong to an earlier period, and is represented by 
two distinct species some time anterior to the appearance of the latter genus. 
* The condition of these fossils is such, that often little more than the form can be ascertained; consequently it is 
difficult to give reliable generic characters, since form is often so variable in species of the same genus. By examining 
the descriptions and figures here mentioned, the reliable characters of these genera will be ascertained; and since 
they are the earliest forms constructed upon this type, they are worthy of consideration, inasmuch as we regard it 
important that the earliest forms should in all cases be regarded as typical species of the genus. 
+ If the figures and descriptions of Euomphalus qualteriatus and Murchisonia gracilis are correct, our species, 
which have been referred to these, are quite distinct. 
