XX INTRODaCTIOJf. 



of England. The Lepfesna 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 Belthyris 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 Leptana and Orthis are far more conspicuous and 

 important than the Mrypa, the species of which are fewer and generally less widely dis- 

 ti il>uted. Of the latter genus, however, two species, the A. modesta and A. increbescens , are 

 abundant and widely distributed, being every where typical of the lower division of the 

 system. 



So few species of Acephala from this period have been described in Europe, that no 

 very satisfactory comparison can be drawn between them and our own. We recognize, 

 nevertlielcss, 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 palaeozoic 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 aiuicipated, being more than half as many as of the Bracliiopoda, 

 and are therefore entitled to our consideration. 



Among tlie Gastroi-ods, 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. f In this class, we have several 

 genera peculiar to Lower Silurian strata. Among tliese may be included Ophilcta, Raphi- 

 stoma, Scalites, Madurea, Bucarda and Cyrtolites. The species in this class, although 

 juuubering several genera, are not numerous till we arrive at the base of the Trenton 

 limestone. The generic form Bucania, wliicli I have separated from forms licrelofore 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 liHlc 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. 



t If the figures and descriptions of Euomphahis qualteriatus and Murchisonia gracilis are correct, our species, 

 which have been referred to these, are (juite distinct. 



