366 REPORT OF THE HARVARD AFRICAN EXPEDITION 
Allen (see Chapter XX XIII), in his study of the rodents of Liberia, notes 
that Mus rattus Linné, the (European) black rat, is occasional but less abun- 
dant than the gray rat. He found examples and notes its occurrence as far 
inland as some fifteen miles in the interior up the Du River. Several specimens 
were collected from native houses. He further noted that Mus alexandrinus the 
gray roof rat, swarms about the native villages, where it lives chiefly in the 
thatched roofs of the houses by day, and at night ventures forth in search of food. 
In many of the houses the rough ceiling formed by cross poles or split palm con- 
stitutes a refuge for rats. Exactly how these rats have reached even remote 
villages, separated from their neighbors by miles of forest, is difficult to see, un- 
less they have been transported by persons carrying baggage, though it is possible 
that they may follow along streams and trails to some extent. He also points out 
that Mus norvegicus is found in Liberia, but that it is much less common than 
the roof rat. 
In all, seventeen species of rats and mice were found in Liberia. Should a 
focus of bubonic plague at some time become established in Monrovia, oppor- 
tunity for its spread would be afforded by the abundant rodent population. A 
species of Xenopsylla was also found in Liberia. Sporadic cases of plague have 
occurred during the past year on the West Coast of Africa, in Senegal, Nigeria, 
and Angola. 
