POTSDAM PERIOD. 



181 



close of the Silurian age the Eadiate type — the lowest of the four — 

 was less fully displayed in its subdivisions than the Molluscan. 



4. Among Articulates: marine worms, and Crustaceans of the tribes 

 of Trilobites (figs. 242, 243, 245) and Ostracoids (p. 154). 



or ovate; a slit or foramen through the ventral valve; no articulation between 

 the valves. 



Lingula Family (figs. 161 and 236). — Thin and small shells; orbicular or sub- 

 ovate; no foramen; no articulation. 



Besides these there are also the Crania and TJiecidium families. 



Gexera of Brachiopods. — 1. Terebratula Family. — Genus Terebratula, like 

 figs. 160 and 208 ; the loop small, as in fig. 209. Genus Waldheimia, the same, 

 the loop large, fig. 208. 



Besides these genera, Terebratulina has the side (or " crural") processes near the 

 base of the loop united (fig. 210). Another genus, Terebratella, has the sides of the 



Figs. 208-215. 



Fig. 208, Waldheimia flavescens ; 209, loop of Terebratula vitrea ; 210, id. Terebratulina 

 caput-serpeutis ; 211, Spirifer striatus; 212, same, interior of dorsal valve; 213, Athyris 

 concentrica; 214, 215, Atrypa reticularis, the latter dorsal valve. 



loop united at middle by a cross-piece, and this piece soldered to the shell. Tere- 

 brirostra has the beak extravagantly prolonged, so as to be longer than the dorsal 

 valve. Rensselaeria has, instead of a loop, a peculiar hastate brachial support, 

 projecting far within the dorsal valve. Striclclandia of Billings may be the same 

 genus, and, if so, it antedates Rensselaeria. Centronella seems to be intermediate 

 between Terebratula and Waldheimia. Other genera, rarely met with, are Tri- 

 gonosemns, Meyerlia, Mayas, Argiope, appearing first in the Cretaceous, and 

 Kramsia, Bouchardia, and Jforrisia, known only in recent seas, with a possible 

 exception of the last. Stringocephalus is another genus, probably constituting 

 a sub-family, occurring in the Devonian. 





