POTSDAM PERIOD. 185 



Besides these remains of Crustaceans, there are peculiar tracks, 

 found at Beauharnois and elsewhere in Canada, and called Protich- 

 nites (fig. 245 B), which are supposed to have been made by large 

 Crustaceans having stout legs like the modern Limulus : they are 

 anomalous in form, and need further explanation. A very different 

 kind of track, also first made known by Logan (fig. 245 A), occurs in 

 the same Canada rocks. It is six and three-quarter inches wide ; and 

 one trail is continuous for thirteen feet. It has been regarded as 

 the track of a very large Gasteropod ; but it is quite as probable 

 that it was made by the clusters of foliaceous appendages of one of 

 the great Trilobites, — these appendages being its locomotive organs. 



Impressions of long marine worms have been reported from some 

 of the shales. Besides these, there are worm-holes in the Potsdam 

 sandstones — though now filled with rock — which are referred to 

 burrowing worms of the Arenicola family (so called from the Latin 

 arena, sand, and incola, inhabitant). They penetrate the rock verti- 

 cally, and are often in pairs, as is now the habit of such worms. The 

 most common kind in the Potsdam sandstone is called Scolithus 

 linearis (fig. 240) ; and for a long time it was supposed to be the 

 remains of a fucoidal plant. They are so abundant in these de- 

 posits of the Potsdam epoch that they serve to identify them in 

 different regions. 



In the North American rocks of the Potsdam period, already 

 nearly sixty species of Trilobites have been found and described, 

 and over forty species of Graptolites. Although the species of 

 shells were not numerous, some kinds were exceedingly abundant, 

 and many layers are almost wholly composed of them. 



If the Potsdam and Calciferous epochs were named from their 

 most characteristic species, the former would be the Lingula or 

 Paradoxides epoch, and the latter the Graptolite. 



wide, having a pointed beak, very large triangular hinge-area, and internally- 

 digitate muscular impressions ,• commenced in the Trias, and has a single living 

 species. 



Davidsonia is a genus of rare occurrence and undetermined relations. There 

 is some resemblance to Lejitsena ; but it has a pair of low and faint spiral cones 

 on the inner surface of the larger valve. 



The following genera have species in the existing seas; and those having an 

 asterisk are known only recent. In the Tcrebratula family, the genera Tere- 

 bratula, Waldheimia, Terebratella, Megerlia, Kraussia,* Bouchardia,* Morrisia, 

 Argiope; in the Thecidium family, Thecidium: in-- the Rhynchonella family, 

 Rhynchonella; in the Crania family, Crania; in the Discina family, Discina; in 

 the Lingula family, Lingula. There are no living species of the Orthis, Pro- 

 ducts, and Spirifer families. 



