150 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
oblique and facing dorsally, the branchial opening irregularly 
notched . Fig. 6 , a, shows its ordinary appearance when expanded . 
At intervals the anal syphon collapses on the branchial one so 
that it becomes almost invisible as shown in b. These contrac- 
FiG. 6. — Yoldia liynatula (Say). 
tions or collapses go on for some time at the rate of twelve a 
minute. The mantle is fringed with papillae as in the preceding 
species; the mantle tubercle is marked and conspicuous. The 
foot did not appear in the specimens I studied but from descrip- 
tions differs in no respect from that of F. sapotilla. 
ARCA PEXATA Say. 
Fig. 7. Length, 50 mm. 
Fig. 7. — Area pexata Say. 
