— 16 — 



ff. Lower edge of pallial sinus not \mited with pallial line; cartilage pits 

 unequal, tliat on tlie left \ alve on a conspicuous projecting tooth; 

 retracted siphons usually not projecting beyond the shell, tip with- 

 out plates ; umbo not nearer the anterior end of the shell, 

 g. Umbo central ; valves rounded and slightly gaping at the posterior 

 (siphonate) end. 



Soft-shell Ml/a urenaria 



p. 56 ; pi. 17 , figs. 2, 3 



gg. Umbo nearer the posterior end of sliell ; valves truncated and widely 

 gaping at posterior end. 



Platyodon Platijodon cancellatus 



p. 62 ; pi. 17 . fig-. 1 



Fig. 17. Mya arenaria. One-half natural siz:^. 

 (A) Interior right valve. (B) Exterior of portion of 

 left valve. 



dd. Shell with anterior half sharply marked off from posterior, roughened and 

 obviously adapted for boring in rock or clay; hinge region protected 

 either by a membrane or by accessory plates. (See Fig. 14.) 

 e. Shell gaping widely at both ends throughout life ; a membrane but no 

 accessory plates covering hinge region. 



PIddock y.irfaca gahbi 



p. 63 ; pi. IS , fig. 1 



ee. Shell of adult closing completely or gaping only moderately, siphons com- 

 pletely retractile, accessory plates present over hinge region, 

 f. Siphonal end of valve formed in large part by soft epidermal scales ; 

 accessory plates long and slender. 



Borer Pavapliolas californica 



p. 64 ; pi. 19 , fig. 1 



f f. Siphonal end of valve with epidermal scales which cover the firm shell ; 

 accessory plate short and triangular. 



Rock clam PhnUtdldea penita 



p. 64 ; pi. 19 , fig. 2 



