142 WILLIAM RAY ALLEN. 



EXPLANATION OF PLATE I. 



The arrows indicate the directions of the ciliary currents. 



The arrows which follow the margins of the inner gills show the direction of 

 ciliary currents in the troughs which form their under edges (Fig. 7, tr). 



x x x x x x indicates the line which divides mouthward-flowing streams from 

 those leading posteriorly. 



In all except Figs. 6 and 7 the left side is posterior and the right anterior. 



FIG. i. (Adapted from Wallengren.) The organs concerned with food-getting; 

 the right shell removed and mantle folded back; showing ciliary streams of mantle 

 and gills. 



FIG. 2. (Adapted from Wallengren.) As in Fig. i; mantle gills and palps of 

 right side folded back; showing ciliary streams of inner gill and body wall. 



Abbreviations. 



A, anterior. P, posterior, 



/, foot, r, mucous accumulations of refuse 



gi, inner gill, material, 



go, outer gill, .s, siphon, 



Ip, labial palps, X, point of convergence of ciliary 



md, right mantle, currents near labial palps. 



ms, left mantle, 



