POTSDAM PERIOD. 



191 



out over the bottom and fixed in the mud only at centre. -They probably 

 grew at considerable depths. In fig. 246, which exhibits the centre of a branch- 

 Figs. 246-252. 

 48 



WBwm 



' T TjTiTTTl 1 1 1 1 1 1 1 1 ) 1 1 1 1 1 1 i 'il 



—•^52 



Figs. 246, 247, 248, Graptolithus Logani; 249, 250, Phyllograptus Typus; 251, Graptolithus 

 pristis ; 252, young of Graptolite. 



ing group, as figured by Hall, a membrane unites the branches at base ; 247, 

 a portion of a branchlet; 248, same, enlarged. The delicate notching along 

 the margin is made by the cells of the several animals of the group. Figs. 249 

 and 250 are a leaf-shaped kind, the type of the genus Phyllograptus (the P. Typus 

 H.). [Fig. 251 is a species from New York, the G. pristis H., from the shales of the 

 Hudson River period.] Fig. 252 is regarded by Hall as a young Graptolite, from 

 the Canada graptolitic shales. Numerous such forms were found among the 

 impressions of Graptolites. 



c. Echinoderms. — Crinoidal remains are not common. Among them Billings 

 has distinguished some stems that probably belong to the genus Glyptocrinus 

 (see fig. 339 for a species of this genus), and a fragment of the head of a Cystid- 

 ean near Palseocystites tenuiradiatus H. of the next, or Chazy, epoch. There are 

 fragments of several other species. 



Mollusks. — a. Bryozoans. — A Stromatopora (a massive coral consisting of 

 thin layers that are made up of minute cells, a species of which is common in 

 the Trenton period) has been found at Phillip sburgh, Canada (Billings). 



b. Brachiopods. — The Lingula family is no longer the most prominent among 

 Brachiopods: the Orthis family takes precedence. Along with it occur species 

 of the Rhynehonella family, as in the Potsdam. 



Fig. 253, Orthis (Orthisina?) grandseva B. Other Brachiopods are Lingula 

 acuminata Con., Orthis parva f Pander, Camerella ealci/era B., a species of 

 Leptsena, and of Strophomena. 



c. Conchifers. — The first of this group yet reported is the Conocardium Blumen- 

 bachii B., found at the Mingan Islands in the White limestone (p. 176), a shell 



another and with the tubular axis. In the fossilized specimens it is often diffi- 

 cult, as would be inferred from the above figures, to determine which is the 

 fact; and hence there are some doubts as to the relations of the Graptolites. It 

 is quite possible that, while most of the so-called Graptolites are Sertularian 

 (that is, Acalephs), some are Bryozoan (or Mollusks). 



