— 14 — 



cc. Shell margins when closed gaping at points where siphon or foot or both are 

 protruded, contracted siphon often projecting beyond closed shell, 

 d. Shell with anterior and posterior portions not obviously differing in surface 

 markings, nor adapted for boring ; hinge region without either mem- 

 brane or accessory plates. (See flg. 13; compare with fig. 14.) 



Fig. 13. Panope gcnerosa. 

 E.xterior left valve. 



One-lTalf natviral size. 



Ligament external, 

 f. Shell not markedly elongated (length not equalling three times the 

 breadth) ; animal never moving freely up and down a permanent 

 burrow, 

 g. Shells not exceptionally thin nor gaping markedly at anterior end to 

 accommodate foot, periostracum not conspicuous, no internal rib; 

 not found in clear sand on open beaches. 



Fig. 14. Zirfaea gahbi. One-half natural 

 size. Exterior left valve. 



h. Valves gaping widely, particularly at siphonate end ; pallial sinus 

 small, not reaching over i of the way to the anterior muscle 

 scar, not united with the pallial line ; siphons united, extremely 

 long and large, covered with thick dark epidermis. 

 Geoduck Panope generosa 



p. 63 ; pi. IS , fig. 2 



hh. "Valves gaping moderately; pallial sinus large, reaching i to i of 

 the way to the anterior muscle scar, united with pallial line; 

 siphons separate, long, without heavy epidermis. 

 Sunset shells Psammohia 



i. Shell oval (about one and a half times as long as wide), umbos 

 nearly central. 



P. californica 



p. 45 ; pi. 14 , fig. 1 



ii. Shell elongated (about twice as long as wide), umbos much 

 nearer anterior end. 



P. edentula 



p. 45 



