15 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



way between the shell margin and the apex of the slit in 

 the girdle; remaining series of setae about four in number, 

 much smaller, all on the outer third of the girdle, those of 

 the more marginal series very minute and more bushy 

 (i. e.. their spines less asymetricahy arranged) than the 

 sutural setse, those of the inner series progressively larger 

 and except for their smaller size and somewhat shorter 

 aciculae similar in general makeup to the sutural setse. 



Ctenidia about 20 on a side. 



Color of tegmental surface of shell a rather light brownish 

 or brick red, with more or less brownish-maroon and gray- 

 buff maculation, the latter color usually prevailing on the 

 third valve and sometimes the seventh as well ; in one speci- 

 men the sides of valve vii being light yellow. Upper sur- 

 face of girdle green in the type, reddish in all other speci- 

 mens. Interior flesh tinted, shading darker in the deeper 

 portions. 



Measurements: Maximum length of type specimen, 20.0; 

 maximum width, 11.2; altitude, 3.7; length of shell only, 

 18.3; width of tegmentum in valve iv., 8.5 mm. 



Type: A specimen preserved dry [S. S. B. 281], entered 

 as Cat. No. 3939 of the author's collection. A paratype 

 collected in 25 fathoms by Willett in 1914 [S. S. B. 285] is 

 deposited in the collection of the California Academy of 

 Sciences, and another taken in 15-20 fathoms in 1917 

 [S. S. B. 641] is in the private collection of Mr. George 

 Willett^ 



Type Locality : 20 fathoms, Forrester Island, Alaska ; 

 George Willett, August 5, 1915; two specimens. 



Remarks: Though ranking among the smaller and less 

 conspicuous forms, close acquaintance reveals M. egretta as 

 one of the most beautiful of all the Mopalias. It is one of 

 the Forrester Island species which Mr. Willett obtained on 

 several occasions, though never in any numbers, and to 

 which I referred on pp. 239-240 of the first paper of the 

 present series. His discovery of another specimen in 1917 

 served to confirm the validity of the diagnostic characters I 



'A specimen in alcohol [284] and a series of dislocated valves [640] have also 

 been drawn upon for portions of the foregoing description and therefore rank as 

 paratypes. These are at present in the author's collection. 



