2SS 



C(ELHEL^IINTHES 

 P 



g at the posterior end for the passage 



th' bears a 



strongly arched and has an opening ul mc pusuruui cuu ivi ui^ jjtic.;.!!^^ 



of the staUi (tigs. 277, 2 78). The rlatter dorsal valve frequently bears a 



-acteristic feature in the calcareous skeleton of the arms (llg. 2 78) which, 



characteristic feature in the calcarec 

 when present, has very different forms 



\Mten closed the ^-ah'es completely 



Fig. 276. — Anatomy of RhynchomTta psittacca (after Hancock).^ a', left, a', 

 right arm; a, opening into the cavity of the arm; d. intestine; c, blind end of the intestine; 

 g, stomach with liver; m, adductors and divaricators of shell; o. cesophagus; p'. p', 

 dorsal and ventral mantle lobes; .•;/, stalk; i, 2, first and second septum, on the second a 

 nephrostome. 



enclose the body. When tliey open the gape is anterior, the posterior 

 parts remaining in contact. Here, except in the Ecardines, a hinge is 

 developed just in front of tlie posterior margin, consisting of teeth in the 

 ventral valve which ht into corresponding grooves in the dorsal. Opening 



Fig. 277. — ITuhllieimia fla-vcsccns (from Zittcl). Shell with arms and muscles. 

 a, arm with fringed border III); c, c', divaricators; (/, adductors; P, hinge process (.the 

 vertical hue shows position of hinge). 



and closing the valves are, contrary to what occurs in Lamellibranchs, 

 active processes, accomplished hv appropriate divaricalor and adductor 

 muscles (fig. 277). These produce scars on the shell, important in the 

 study of fossil forms. 



