482 DAVIDSON BLACK 
Bdellostoma, though this is not the case in either Petromyzon 
or Myxine. 
The number of gill sacs in petromyzonts is seven, while in 
Myxinidae the gill sacs seldom exceed six pairs, though in rare 
cases there may be seven pairs present (Bridge, 12, p. 422, p. 
X. Sp.oce. 
Fig. 8 Reconstruction charts to illustrate the reduction of the rostral end 
of the caudal viscero-motor column in Bdellostoma and Myxine. (A), Petro- 
myzon fluviatilis (Kappers, 66), (B), Bdellostoma dombeyi, (C), Myxine gluti- 
nosa (Réthig and Kappers, 88). Signs and abbreviations as before. 
281). In Bdellostoma dombeyi, however, the usual number of 
gill sacs is eleven to twelve, though variations both above and 
below this are not infrequent (Worthington, 99). 
With almost double the number of gills to innervate, it is 
not surprising to find the caudal viscero-motor column in Bdellos- 
toma almost double the length of the homologous structure in 
Myxine. It is, however, difficult to understand why the caudal 
