l88 AN INTRODUCTION TO THE STUDY OF FOSSILS 



when adult the pedicle may be lost, but in all such cases the 

 animals are held in place by social crowding, spines, or other 

 devices (Rafinesquina, Stropheodonta demissa, Productus) ; 

 at times it is cemented to some foreign object (Crania, Meekella, 

 Richthofenia). 



he. f umA 



Fig. 74. — Spirifer increbescens Hall, from the Mississippian of Alberta, (x 2.) 

 a-b, length of shell (from hinge line beneath the beak to front of shell) ; b, anterior 

 margin or front of shell; be., beak, the posterior end of the shell; bv., brachial 

 valve; c-c', width of shell; c and c' are the cardinal extremities ; ca., cardinal 

 area of pedicle valve, the flattened area above the hinge line and between the 

 cardinal extremities (each valve has a cardinal area) ; d-e, height of shell (measured 

 between the centers of the valves and at right angles to the plane of length and 

 breadth) ; del., delthyrium, or triangular pedicle opening ; gr.l., concentric growth 

 lines marking the anterior margin of the shell at successive periods ; h-h', hinge 

 line (compare Fig. 78, hi.) ; m.f., median fold (composed here of four plications in 

 this species) ; pi., plications (folds radiating from the beak, not longitudinally 

 striate) ; p.v., pedicle valve; nmb., umbo of pedicle valve. 



For terms describing the exterior of the shell see Fig. 74. 

 The valves are opened and closed by muscles, the impressions 

 of which show upon the interior of the valves in front of the 

 beaks (Fig. 73 a). For formation of ribs, etc., see p. 220. 



The interior of the shell is lined by the mantle, — a mem- 

 branous fold of the body-wall, which is often studded with 

 minute tubes, the caeca; these fit into correspondingly mi- 

 nute holes (tubules) in the shell ; the tubules give to the shells 



