1977] 
Porter — Mesostenines 
77 
The Valley is a comparatively humid refugium surrounded by 
more arid habitats and located near the southwest limit of inter- 
glacial expansion for North American Neotropic species and close 
to the northeast limit for most Middle American species. Its Neo- 
tropic mesostenines are thus of complex distributional affinities. 
Four species, Cryptanura compaeta, C. lamentaria, Lymeon leu- 
cosoma, and Polycyrtidea limitis, are Middle American and range 
from the tip of Texas to Central America. Cryptanura vallis, Bi- 
cristella texana, Diapetimorpha sphenos, D. aspila, and D. pareia 
are apparently endemic to the Valley but almost certainly will be 
found also in Mexico when that poorly known fauna has been 
better collected. Acerastes pertinax and Pachysomoides stupidus 
extend all the way from Brasil to the southeastern U.S. On the 
other hand, Diapetimorpha picta, Mallochia agenioides, M. fron- 
talis, Lymeon cinctiventris, Messatoporus discoidalis, and Agono- 
cryptus discoidaloides are mainly eastern North American and 
reach their southern limit in the Valley. Diapetimorpha macula, 
D. introita, D. acadia, and Lymeon orbus likewise are centered 
in the Atlantic and Gulf states but range south a variable distance 
into Mexico and Pachysomoides fulvus extends over the entire 
U.S. and well into Mexico. Obviously, Pleistocene alteration of 
glacials and interglacials, as well as wetter and drier epochs within 
interglacials, have produced in the Valley a multiple overlap of 
northern and southern Neotropic mesostenines. 
Valley mesostenine genera show extremely wide affinities. Crypt- 
anura, Diapetimorpha, Mallochia, Lymeon, Acerastes, Pachysom- 
oides, Messatoporus, and Agonocryptus range from the northeast- 
ern U.S. to subtropical Argentina, Polycyrtidea from Texas and 
Florida to Argentina, and Bicristella from Texas to Argentina. 
The 11 Neotropic mesostenine genera represented in the eastern 
U.S. all reach Argentina. Of the 34 genera found in Middle Amer- 
ica, 33 cover at least a large part of South America also, while only 
one is endemic. About 20 more Neotropic mesostenine genera are 
endemic to South America. The Neotropic mesostenines thus seem 
to have evolved in South America during the Tertiary and to have 
spread northward massively in those warmer and wetter times. The 
modern Middle American fauna, generically, is a vast sample of 
the South American and the North American fauna a decimated 
vestige of the same stock. In the Tertiary, when humid forests 
covered much of North America below 40 degrees N. Lat. and 
