608 K. MITSUKURI. 
it to possess the primitive gill. On the other hand, Peck 
investigated forms simpler than Unio, and arrived at the 
probably true conclusion. Posner simply began where 
Peck ended. The two investigators, therefore, supplement 
each other, and now, with the addition of the extremely 
simple gills of Nucula and Yoldia, the series is fairly com- 
plete, and it seems to me that the filamentary character of 
the primitive Lamellibranch gill is made tolerably certain. 
