POTSDAM PERIOD. 179 



large as the finger; the species is the P. tubularis. A branched or palmate spe- 

 cies is the Buthotrephis antiqua H. Other species of Palaeophycus occur in the 

 beds of the Calciferous epoch. 



It is possible that the infusoria called Diatoms, now referred to the vegetable 

 kingdom, existed in the waters, and along with Sponges they may have been 

 a source of part of the chert and quartz in the beds. But no remains of these 

 microscopic species have yet been detected in so ancient strata. 



The cylindrical upright stems, called Scolithus, common in the Potsdam beds, 

 are now regarded as the fillings of worm-holes. 



2. Animals. 



Among the animals of the Potsdam or Primordial period, though 

 few in species, three of the sub-kingdoms were represented, — the 

 Radiate, the Molluscan, and the Articulate. 



The species are all marine ; none are proved to be of fresh-water 

 or terrestrial life. They included, — 



1. Among Protozoans : probably Sponges (fig. 236 A) and Rhizopods. 



2. Among Radiates : Crinoids, of the order of Echinoderms ; Grap- 

 tolites, supposed to be of the order of Acalephs ; and possibly coral- 

 making Polyps. 



3. Among Mollu&ks: Bryozoans, Brachiopods,* Conchifers, Ptero- 



* As Brachiopods are the most abundant fossils of the Silurian, their distin- 

 guishing characteristics and the more important genera are here mentioned, 

 — taken principally from Davidson (Palaeontographical Society publications). 



1. Animal. — As stated on page 158, the living animal, unlike all o'ther Mol- 

 lusks, has (1) a pair of spiral arms, as shown in figs. 212 and 215; and to this 

 the name Brachiojpod alludes, from the Greek for arm and foot. These arms 

 may sometimes be thrown far out of the shell, so as to be used for taking food. 

 (2) The animal, as well as shell, is symmetrical either side of a vertical line let 

 fall from the centre of the hinge, — the line a b in fig. 213; and in this the species 

 differ from all the Conchifers (or Lamellibranchs). (3) There are no branchiae 

 (gills) apart from the pallium or mantle; and hence Brachiopods are often 

 called Palliobranchs, 



2. Shell. — The characteristics of most importance are as follow : — 



a. The large valve (see fig. 211 and others) is the ventral. 



b. The form of the internal supports connected with the spiral arms varies 

 much, and often they are wanting. The loop-form is seen in figs. 208, 209, 210; 

 the spiral, in figs. 212, 215; the short process, in fig. 217; and they are wanting 

 in figs. 220, 221. 



c. The general form and exterior markings of the shell afford important 

 characters ; the nearly equal convexity of the two valves, or a medial depression 

 on the ventral valve, with a corresponding elevation on the dorsal, figs. 211, 

 213. 



d. The beak of the shell may be very large and full (figs. 211, 229), or very 



