436 Mr. O. Thomas on a 
This specimen, like others collected at Goya by Mr. R. 
Perrens, has the characteristic whitening of the posterior 
abdomen on which the name was founded. The wings are 
also narrowly edged with white. The next species has the 
abdomen brown throughout. 
5. Myotis nigricans, Wied. 
&,32. Villa Rica. February. 
33,19. Sapucay. January and February. 
These specimens differ from M. albescens by their uniformly 
dark bellies and other details, and are evidently specifically 
distinct. Rengger supposed the two forms to be identical. 
Motossvs. 
Mr. Foster obtained no less than six species of Molossus, 
using this name in the larger sense employed by Dobson. 
The study of these convinces me that the dental formula in 
this group, however convenient as an index to determination, 
is emphatically not a true guide to mutual relationship. 
Those who wish that genera should be really genetic groups 
should not therefore use “ Promops,’ “ Myopterus,’ and 
Molossus as separate genera, unless skull-structure and not 
dental formula is made their basis. 
To commence with: M. Fosteri, with the external pecu- 
liarities of rufus, therefore of true Molossus (s.s.), has four 
lower incisors as in Promops and Myopterus; while as regards 
its small upper premolar, of six specimens, three have got it, 
as in “ Promops,” and three have not, as in Molossus and 
Myopterus. The same variation occurs in M. nasutus. 
Then it is quite evident that the broad-faced flat-heade. 
species cerastes, planirostris, and Temminckii form a natural 
group quite apart from the others. But the last-named has 
a different number of lower incisors to the first two, while 
maurus (really, I think, more closely allied to “‘ Promops” 
abrasus and glaucinus) has the dental formula of “Myopterus”’ 
planirostris and cerastes. 
Mr. Miller is now engaged on a revision of the group, and 
from his use of “ Promops” in describing new species, I 
presume he intends to use that as a genus. If this is the 
case, I shall be interested to see both how he deals with 
Molossus nasutus and Fostert, and whether, if he takes 
Temminckii from the planirostris-group and puts maurus into 
it, he will still be prepared to maintain that his genera have, 
as they should have, genetic relationship as their basis. 
The following is a key, by skull-characters only, to the 
species in Mr. Foster’s collection :— 
