56 PALEONTOLOGY 



remarkably grotesque. Lower valves of this genus and 2%*- 

 ciclium are not uncommon, attached to the tests of sea-urchins, 

 in the chalk ; but upper valves are scarce, either detached or 

 in situ. 



The Discinidce are also ancient fossils, few in number, but 

 appearing in every period. Some of the palaeozoic Discince 

 (= Orbiculoidea, d'Orb.) cannot be generically distinguished 

 from the recent species by any characters with which we are 

 as yet acquainted ; but others (= Trematis, Sharpe) are orna- 

 mented with quincuncial punctures, and the casts exhibit 

 indications of diverging internal plates, which imply very 

 considerable difference in the organization of the animal. The 

 genus Siphonotreta (Verneuil), peculiar to the Silurian forma- 

 tions, is covered with moniliform tubular spines. 



Lingula, which has given its name to one of the oldest fos- 

 siliferous rocks, is another form occurring unchanged in strata 

 of every period. Only 34 species are known, and none of them 

 are very common. The latest British Lingida is found in the 

 coralline crag (older pliocene) of Suffolk : the nearest living- 

 species is as far off as the Philippines. L. 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 ; externally it has all the appearance of an ordi- 

 nary existing shell. From the fragments of Lingula in the 1< twer 

 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 Russia 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 presence of phosphate of iron. 

 In shape it is regularly oval, and differs from Lingida in the 

 character of the interna] must ular impressions. 



