1908.] NATURAL SCIENCES OF PHILADELPHIA. 475 



Zone IV (20°-15° C, 68°-59° F.), the Mexican Tierra Templada of 

 Hann, embracing a large part of the United States, nowhere touches 

 the coasts in Mexico or Central America, as Zones II and III do, but 

 occupies a central position. It consists in these countries of a Mexican 

 portion, of rather greater area than that of the central plateau, and 

 reaching to southern Puebla ; a mostly elongated and narrow strip in 

 Guerrero and Oaxaca, some of the western parts of Chiapas, Guatemala 

 and Honduras, with an arm into western Salvador; a number of small 

 scattered areas in Honduras and a larger one in northern Nicaragua ; 

 finally, an elongated strip in Costa Rica and Panama. Representatives 

 of Zone IV are Cordulegaster godmani, Mshna dugesi, Plaihemis suhornata, 

 Lihelhda comanche, foliata, nodisticla and luctuosa, members of chiefly 

 northern, or {.^shna) cosmopolitan, genera. 



Zone V (15°-10° C, 59°-50° F.), the Mexican Tierra Fria of Hann, 

 occurs in scattered areas in Sonora, Chihuahua, Durango, Zacatecas, 

 and one of greater extent in the States of Hidalgo, Puebla, Tlascala 

 and Mexico ; in western Guatemala and on some of the peaks of Costa 

 Rica. No Odonata are peculiar to this zone. 



Zone VI (less than 10° C. = 50° F.) occurs only on the higher peaks 

 of Mexico, Guatemala and Costa Rica; no Odonata have been reported 

 as yet at or above the elevations corresponding to its lower limit in 

 these countries. 



Zones III, IV, V and VI may be compared to continents or islands, 

 lying within a sea of Zone II, and each enclosed by a girdle of zones 

 of lower numbers than itself. 



Table 9 gives the distribution of the Odonata within these zones. 



From Table 9 it results that the number of species confined to one 

 zone only is 104, extending through two zones 99, extending through 

 three zones 71, and through four zones 8. Of all the zones. III con- 

 tains the greatest number of species, subspecies and varieties, viz.: 

 222 as compared with 165 in II, 143 in IV, and 10 in V; it is also the 

 richest in zonal endemic species, viz. : 46, as compared with 40 in II 

 and 18 in IV; and the richest in endemic Mexican and Central Ameri- 

 can forms, viz. : 106, as compared with 60 in IV, 56 in II, 4 in V, and 

 3 in I. 



The species, etc., which, outside of Mexico and Central America, 

 are found exclusively in Northern America appear in the tempera- 

 ture zones of our district in the following numbers: IV 15, III 13, 

 II 5, V4. As our map (Plate XXVI) shows, zones IV and III are con- 

 tinuous from the United States into Mexico, so that they offer a 

 pathway for the extension of species whose living conditions are 



