A text-book of geology, intended as an introduction to the study of the rocks and their contents - Softcover
Synopsis
This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1897 Excerpt: ...of the Eocene Period, they have played an important part in the formation of the stratified rocks. Sub-kingdom II.--Porifera, or Spongida.--The soft parts of many sponges are supported by rays or spicules composed of siliceous or calcareous matter; and it is these spicules only which we find fossil. The oldest known British fossil sponge is Protospongia fenestrata, which occurs in the Lower Cambrian strata at St. David's in South Wales (in the Menevian Beds). Sub-kingdom III.--Coelenterata.--The Coelenterata (Greek cadus, hollow; enteron, intestine) are divided into two classes--the Hydrozoa and the Actinozoa. The Hydrozoa include the jelly-fishes and other forms which have no hard parts, and which we cannot, therefore, expect to find as fossils. But in this class we have also the important extinct order of the Graptolites. They consist of numerous cells arranged on one or both sides of an axis, which may be either straight or curved. Each cell contained a small organism, so that the entire structure was that of a compound zoophyte, something like the "sea-firs" of the present day. Fig. 94.--Piece of Shale containing Graptolites. GRAPTOLITES AND CORALS. 186 Graptolites are found only in the Upper Cambrian, Ordovician, and Silurian strata. The name is derived from grapho, I write; and lithos, a stone; because shaly rocks covered with these fossils look as if they had been scribbled upon (see fig. 94). The first graptolite belongs to the genus Dictyograptus, and occurs in the Lingula Flags. It is composed of a great number of radiating branches connected together by minute cross-rods, thus forming a network. Didymograptus Murchisoni is a well-known Llandeilo graptolite, consisting of two branches united together to form a V, and having the cells on ...
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