will 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 1 7 Upper Tertiary species are 

 still living on our sea-bottoms ; and many recent Mediterranean 

 forms occm- 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 owino- 

 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 Discyina to the flattened tongue- 

 shaped Lingula. The valves have most commonly a smooth 

 external 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 {Si'phonotrcta), 

 which perhaps served to anchor it in the soft mud of the sea- 

 bottom. The usual mode of fixation was by means of the pedicle 

 ( = peduncle or stalk), which either (1) passed out simply between 

 the posterior gaping portion of the valves {Lingida), or (2) lay 

 in a slit in the ventral valve {Lingulella), 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, 

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

 the whole surface of the ventral valve (Crafiia, p. 467). 



The two valves in the fossil Ecardines were held together by 

 muscular action, though in some families {Trimerellidae) 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 genei- 

 ally smoother than other parts of the valves, more or less tii- 

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

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



