XVIII ECARDINES: EXTERNAL CHARACTERS 493 



both divisions rapidly increase until in the united Ordovician 

 and Silurian there are nearly 2000 species and about 70 genera. 

 From this point of maximum development down to the present 

 day there is a gradual decrease in numbers. 



According to Davidson, at least 17 Upper Tertiary species 

 are still living on our sea-bottoms; and many recent Mediterra- 

 nean forms occur in the Pliocene rocks of the islands and shores 

 of that sea, and in the Crags of East Anglia. 



A brief review of the chief characteristics of fossil Brachio- 

 poda is given below. Those genera which have the greatest 

 zoological or geological importance can alone be noticed owing 

 to the exigencies of space. 



I. ECAEDINES 

 External Characters 



A considerable diversity of external form is met with even in 

 this division, from the limpet-like Discina to the flattened tongue- 

 shaped Lingula. The valves have most commonly a smooth ex- 

 ternal surface with delicate growth-lines ; but sometimes pittings 

 (^Trematis') or radiating ribs (^Crania) are present, and in a few 

 forms the shell is furnished with spines QSiphonotreta)^ which 

 perhaps serve to anchor it in the soft mud of the sea-bottom. 

 The usual mode of fixation was by means of the pedicle (= pe- 

 duncle or stalk), which either (1) passed out simply between the 

 posterior gaping portion of the valves (^Lingula) ^ or (2) lay in 

 a slit in the ventral valve (^Lingulelld)^ or (3) pierced the sub- 

 stance of the latter valve by a definite foramen (^Discina). The 

 first-mentioned condition of the pedicle seems the most primitive. 

 Rarely the pedicle was absent, and the shell was attached by the 

 whole surface of the ventral valve QCrania^ p. 467). 



The two valves in the fossil Ecardines were held together by 

 muscular action, though in some families QTrimerellidae) we see 

 traces of articulating processes. The "hinge line," or line along 

 which the valves worked as on a hinge, is in most forms more or 

 less curved. A "hinge area" (i.e. that portion of the shell gen- 

 erally smoother than other parts of the valves, more or less tri- 

 angular in form, and lying between the beaks on one or both 

 sides of the hinge line), is usually absent in the Ecardines. 



