BRACHIOPODA. 649 



Silurian systems have yielded nearly two thousand species. Vast as 

 is the number of Ordovician forms, the Silurian rocks are still more 

 rich in this group of fossils ; and the class of the B?-achiopoda may 

 be considered as attaining its maximum in the Silurian period, which 

 has, for this reason, been not unaptly spoken of as the " Age of 

 Brachiopods." In the Devonian rocks more than thirteen hundred 

 species of Brachiopods are known, and though many of these be- 

 long to genera which were previously in existence, others belong to 

 types, such as Uncites, Stringocepha/us, Davidsonia, &c, which are 

 peculiar to this system of rocks. The great genus Terebratula has 

 its first representatives in the Devonian. In the Carboniferous rocks 

 the number of species has undergone a marked reduction, the prin- 

 cipal forms belonging to the great Palaeozoic families of the Stropho- 

 menidce, Spiriferidce, and Productidce. The genus Producta attains 

 here its maximum, some of the species having an enormous geo- 

 graphical extension. In the Permian rocks only about thirty species 

 of Brachiopods are known, and these belong mostly to the families 

 of the Productidce and Strop ho menidce. 



With the beginning of the Mesozoic period, the decay of the 

 Brachiopodous " phylum " which set in during Devonian times be- 

 comes still more pronounced. The Triassic rocks, like the Permian, 

 exhibit a singular poverty in the remains of Brachiopods, but the 

 group is fairly represented in both the Jurassic and Cretaceous 

 deposits. Upon the whole, however, the reduction of the number 

 of species which is so marked in the later Palaeozoic rocks is pro- 

 gressively continued through the Mesozoic period. Moreover, some 

 of the most characteristic of the Palaeozoic families (the At?-ypidce, 

 Trimerellidce, Prodnctidce, and ObolidcE) have totally disappeared ; 

 while the great families of the Strophomenidcz and Spiriferidce are 

 only represented by a few surviving types, and disappear wholly in 

 the Jurassic rocks. On the other hand, the families of the Terebra- 

 tulidce and Rhyncho?iellidce now assume a marked predominance ; 

 the latter, however, being almost exclusively represented by species 

 of the genus Rhyncho?iella itself. Finally, in the Tertiary rocks 

 Brachiopods are few in number, and belong almost wholly to exist- 

 ing genera, while a number of the species still survive. 



As regards classification, the Brachiopoda may be divided into the 

 two orders of the Inarticulata and the Articulata, the characters and 

 families of which will be briefly treated of in what follows. 



Order I. Inarticulata 

 ( = Tretenterata, Lyopomata, Pleuropygid). 



This division of the Brachiopoda comprises those forms in which 

 the valves of the shell are not hinged, but are held together by 



