G2 PALEONTOLOGY. 



Davisii, of the "lingula flags" in North Wales, has a pedicle 

 groove in the ventral valve, by which the posterior adductor 

 (or cardinal muscle) must have been divided into two elements, 

 as in the genus Obolus (fig. 13, 7); externally it has all the 

 appearance of an ordinary existing shell. From the fragments 

 of Lingula in the lower Silurian stiper stones of Shropshire, 

 they appear to belong to a species distinct from L. Davisii. 

 Obolus, Eichw. (== Ungula, Pander) is so abundant in the 

 lower Silurian sandstones of Sweden and Eussia as to have 

 given its name to the " obolite grit." In England it occurs 

 only in the upper Silurian of Dudley. The shell is horny in 

 texture, and often stained blue, like "the Lingula, by the pre- 

 sence of phosphate of iron. In shape it is regularly oval, and 

 differs from Lingula in the character of the internal muscular 

 impressions. 



Class II.— LAMELLIBKANCHIATA* 



This class is so named because the breathing organs (fig. 

 14, p)_ are shaped like leaves or plates, two on each side, de- 

 pendent from the inner surface of the mantle-lobes (a, 0). The 



Fig. 14. b 



Psammdbia florida. 



mouth is provided with ciliated tentacles (ib. h) usually much 

 shorter than in the Brachiopoda, like which the present class is 

 acephalous. A few genera are fixed either by a soldered valve 



* Lamella, a plate ; branchia, a gill. 



