138 THE NAUTILUS. 



faint, but marked by a recession of the ventral border of the valves, 

 Avhich gape but very little and not at all in front ; anterior end 

 rounded, but smaller than the posterior ; ventral border arcuate ; 

 hinge and pallial sinus much as in the last species, except that the 

 sinus is somewhat smaller and less depressed. Lon. 120, alt. 93, 

 diam. 50 mm. 



Distribution : San Diego, Hemphill and Cooper. 



This fine and perfectly distinct species appears rare and I have 

 seen but two specimens, both from San Diego. 



The preceding species belong to the subgenus Standella as 

 adopted by H. and A. Adams, but the following is a true Mactra, 

 ■with the ligament separated from the cartilage pit by a shelly plate. 

 Mactra dolabriformis Conrad, 1S67. PI. V. fig. 1. 



Shell much compressed, polished white under a dull brown epi- 

 dermis, subequilateral with inconspicuous beaks. It closely resem- 

 bles M.Jalcata Gould (from type) but has higher beaks more cen- 

 trally set, the anterior end more attenuated and less truncate, the 

 left anterior lateral tooth single and distally more prominent; the 

 left cardinal larger and wider ; the posterior adductor scar horizon- 

 tally elongate and smaller. Lon. 90, alt. 63, diam. 26 mm. 



Distribution : San Diego, Cala. to Guaymas, Mexico ; " Panama" 

 Conrad. 



This remarkably handsome shell has not unnaturally long been 

 •confounded with M.faleata, from which the hinge separates it sub- 

 generically. 



The true M. califoruica of Conrad is a Mactrmula and can at 

 once be recognized by its sulcate beaks. It reaches 36 mm. in 

 length. The M. planulata is also a small species, resembling M. 

 pobjnyma in miniature. The northern form generally referred to 

 M.Jalcata is a barely separable variety of 31. poly ny ma which may 

 take the name of Alaskana. 



NOTICES OF NEW CHITONS, III. 



BY H. A. PILSBRY. 



Certain rectifications of the previously accepted nomenclature 

 have become necessary, and may be made here. 



