666 
Joumal of Agricultural Research 
Vol. XXVIII, No. 7 
STRONGYLIDAE OF THE HORSE, DONKEY, MULE, AND ZEBRA 
Key to the genera. 
1. Mouth capsule greatly elongated; external leaf crown contains only six 
elements and the two occupying the lateral positions are broader than 
the four occupying the submedian positions_Cylindropharynx, p. 668. 
Mouth capsule not greatly elongated; external leaf crown elements usually 
more numerous than six, and all similar in size and shape_2. 
2. Teeth present in oesophageal funnel_3. 
No teeth present in oesophageal funnel_5. 
3. Teeth of oesophageal funnel do not project into mouth capsule; bursa 
bilobed, that is, no median lobe present, and no median dorsal ray; pre- 
bursal papillae short; spicules terminate without hooks but inclosed in a 
conspicuous sheath_Oesophagodontus, p. 667. 
Teeth of oesophageal funnel project into mouth capsule; bursa trilobed, 
that is, a median lobe present and a median dorsal ray; prebursal papillae 
long; spicules terminate with hooks and without a conspicuous sheath._4. 
4. Oesophageal funnel very large and hemispherical, with three pairs of teeth 
at its base and three large teeth projecting from its walls into the 
posterior part of the short cylindrical mouth capsule; internal leaf 
crown elements large and stout; ventral and lateral rays of bursa arise 
from a common stem_Gyalocephalus, p. 667. 
Oesophageal funnel small and comparatively inconspicuous, possessing 
three large teeth, each consisting of two plates, which project far into the 
large, hemispherical mouth capsule; internal leaf crown elements short 
and relatively inconspicuous; ventral and lateral rays of bursa do not 
arise from a common stem_Triodontophorus, p. 667. 
5. Mouth capsule large, cup or goblet-shaped; dorsal gutter strongly developed, 
forming a conspicuous thickening along dorsal wall almost or entirely 
to anterior margin of capsule; teeth may be present as projections of 
the walls of the capsule in its posterior part_6. 
Mouth capsule comparatively small, more or less cylindrical; dorsal gutter 
seldom well developed; walls of mouth capsule never give rise to teeth._7. 
6. External leaf crown consists of very numerous slender elements; teeth often, 
though not always, present in mouth capsule; dorsal and externo-dorsal 
rays of bursa arise from a common trunk; spicules do not terminate in 
hooks_Strongylus, p. 667. 
External leaf crown consists of only a few large, broad elements; no teeth 
in mouth capsule; ventral and lateral rays of bursa arise from a common 
trunk; spicules undescribed_Craterostomum, p. 668. 
7. Internal leaf crown elements large and broad, sometimes not all of equal 
length; externo-dorsal and dorsal rays arise from a common trunk; this 
main dorsal trunk split only about half-way to the base, so that the 
lateral branches arise from the undivided portion; vulva and anus of 
female comparatively far apart_Poteriostomum, p. 667. 
Internal leaf crown elements vary in size and shape in different species; 
externo-dorsal and dorsal rays do not arise from a common trunk; main 
dorsal trunk split more than half way to the base, so that one or both 
pairs of lateral branches arise from the divided portions; vulva and 
anus close to each other_Cylicostomum sensu lato, p. 668. 
