Figures 1-6. Nematodirus spp. of North American domesticated ruminants. Diagrammatic drawings of anterior and cervical regions; right lateral views showing cephalic expansions, number and pattern of distribution of cuticular ridges, and positions of cervical papillae (cp), excretory pore (exp), and distal end of esophagus (es). 1. N. filicollis. 2. N. davtiani. 3. N. helvetianus. 4. N. oiratianus interruptus ssp. n. 5. N. abnormalis. 6. N. spathiger.
(Lichtenfels & Pilitt, 1983, Proc. Helminthol. Soc. Wash. 50: 261-274)