BEACHIOPODA. 6 1 



guished from Producta (ib. 4) by a row of spines along the 

 hinge-margin of the convex-valve ; it also has a narrow hinge- 

 area with a covered notch, and small hinge-teeth. There are 

 25 species in the Silurian and carboniferous strata, usually of 

 small size, and finely striated. 



In the order Lysopomata* the bell-valves are inarticu- 

 lated, and in most are not completely calcified. Crania is 

 one of the oldest living types, ranging upwards from the 

 lower Silurian. One of the earliest species appears to have 

 been unattached, and another to have had hinge -teeth. 

 Crania Igndbergensis, of the chalk of Sweden, has the valves 

 externally alike, being attached only when very young. The 

 internal markings of C. antiqua, and other fossil species, are 

 remarkably grotesque. Lower valves of this genus and of 

 Thecidium are not uncommon, attached to the tests of sea- 

 urchins, in the chalk ; but upper valves are scarce, either 

 detached or in situ. 



The Discinidai are few in number ; many are of ancient 

 date, but they appear in every period. Some of the palaeozoic 

 Biscince (= Orbiculoidea, d'Orb.) cannot be generically distin- 

 guished from the recent species by any characters with which 

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

 ornamented with quincuncial punctures, Hnd the casts exhibit 

 indications of diverging internal plates, which imply very 

 considerable difference in the organization of the animal. The 

 genus Siphonotreia (Yemeuil, fig. 1 3, 8), peculiar to the Silurian 

 formations, is covered with moniliform tubular spines. 



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

 fossiliferous 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 Lingula is 

 found in the coralline crag (older pliocene) of Suffolk ; the 

 nearest living species is as far off as the Philippines. L. 



* Gr, lyo, to loosen ; poma, valve. 



