JCKES-BKOWNE : SYNOPSIS OF THE VENKRID^. 83 



Ch. petiti, Desh. (;is Saxidomus] =z Venus rigida, Gould, and 



'Tapes diversa, Sow.). 

 Ch. staminea, Conrad (= Venus mundulus, lleeve). 

 'Tapes orbella, Carpenter. 

 T. laciniata, Carp. 

 T. teyierrinia, Carp, (the type of Callithaca, Dall). 



Of these eight species I consider the first to be a typical Cliione, for 

 it has the teeth of Chione with the left posterior parallel to the 

 nymph, and it may he regarded as the Pacific analogue of tlie 

 Caribbean Ch. granulata., which Dr. Dall himself classes as a Chione. 

 The last species on the list is veiy different from all the others, and 

 I regard it as a form of 'Tapes. The remaining six species do form 

 a special group with characters of their own ; they differ from Chione 

 in the features already mentioned, as well as in the greater depth of 

 the pallial sinus. The sub-genus of Tapes which they most nearly 

 resemble is Ruditapes {7'. decussata), but from this they differ in the 

 following particulars, i.e. in having — 



1. A stronger hinge-plate with a broader anterior expansion. 



2. Longer and stronger teeth. 



3. Smooth nymphs, never corrugated. 



4. Left anterior tooth entire, not grooved. 



5. Crenulated valve-margins. 



6. Pedal scar confluent with adductor. 



Tiie fact is that the Protothaca group has characters which make 

 it inconvenient to include it either under Venus or under Tapes, and 

 1 therefore propose to consider it a genus, especially as I believe the 

 following species may also he referred to it, Chione jedoensis, Lischke, 

 Ch. hirasei, Pilsbry, Ch. costata, Q. & G., and possibly the shell known 

 as Petricola elliptica, Sow. (from Peru). The group is essentially 

 a Pacific one, and may be defined as follows : — 



Shell oblong, of dull white, yellow, or brownish colouring, sculpture 

 more or less cancellate, but the radial ribs often becoming dominant. 

 Lunule defined, but escutcheon absent, or only defined in the left 

 valve. Ligament very long and prominent. Hinge-plate strong and 

 deep; teeth separate and rather near together, both medians bifid, 

 and both posteriors more oblique than in Chione. Nymphs smooth. 

 Pallial sinus fairly deep and rounded in Californian species, short and 

 subangular in others. Pedal scar confluent at top with that of the 

 adductor. Ventral margin crenulated, but often becoming smooth 

 posteriorly, llidge and groove on dorsal margins very short. 



Dr. Dall describes the animal of the type as having short siphons 

 which are united to their tips, the foot hatchet-shaped (? linguiform), 

 and not byssiferous nor exhibiting even a byssal groove. 



There is another shell which I am inclined to place under Protothaca 

 in spite of the fact that its margins are entirely smooth, and that it 

 was placed under ' Alarcia ' by Dr. Dall. This is the Venus rufa, 

 Lam., a large, thick, oval shell which has a curiously curved hinge- 

 plate and teeth, which are quite different from those of Samarafigia. 



