Three parts of this paper dealing with the Nereidiformia of the "Albatross" collections have been published in these Proceedings as follows: I, 1909, pp.321-351, Plates XV and XVI; II, 1910, pp.328-402, Plates XXVIII-XXXIII; III, 1911, pp. 234-318, Plates XV-XVI. The long delay in the appearance of this fourth and concluding part is due to several unavoidable circumstances. The paper was practically completed in 1911, but before submission for publication the entire manuscript including figures was unaccountably lost. For a long time, the author was reluctant to believe that it would not be recovered, and took no steps to replace it. Later, it became completely occupied with other work, and only recently had found time to rewrite it.
The text has now been prepared chiefly from the original notes, but supplemented by some additional studies. Some of the drawings have been replaced, and appear on the plates, but unfortunately it has been impossible to redraw all of the figures and as a result this part is less fully illustrated than the others.
In the meantime, several of the species named as new in the original MS. have been descirbd by others, as referred to in the text. The types of all new species are in the collection of the U.S. National Museum, and a set of cotypes and duplicates is deposited at this Academy.