458 ARABIAN NEMATODES, 



(4.) By averaging the specific formulae of a genus, we may 

 obtain a generic formula. During phylogenetic and 

 systematic studies the specific and generic formulse 

 greatly facilitate the necessary comparisons. 



I hope by the aid of this new formula to be able to describe 

 such species as belong to already well known genera, even without 

 the aid of illustrations, so accurately as to leave little to be desired, 

 and yet so briefly as to leave space for the full discussion of biolo- 

 gical and economic problems. 



Oncholaimus, Duj. 



O. ORiENTALis, Cobb.* fH -s ^'2^ '1 t~^-2 ^'^ °"°- Submedian haii's, 

 so inconspicuous as to escape observation under ordinary magnifi- 

 cation, may be found throughout the length of the body by use of 

 the highest powers. The cuticula is not striated. To the convex- 

 conoid anterior half of the neck succeeds a somewhat compressed 

 head, which is rounded in front, and bears, well forward, a row of 

 six slender setse. The six large and thin lips form a kind of dome 

 over the pharyngeal cavity, their free extremities appearing, how- 

 ever, never to meet. Each lip therefore approximates in shape to 

 an isosceles spherical triangle. Chitinous thickenings strengthen 

 the margins of each lip, and from the apex a movable incurved hair 

 or flap projects forward and inward. About a dozen minute nuclei 

 occur near each point where two adjacent lips join each other at 

 the base, — nuclei very much smaller than those found elsewhere 

 in the body, being in fact scarcely larger than the oesophageal 

 pigment granules. Doubtless these nuclei appertain to the 

 muscles and nerves which govern the lips. I saw no papillae 

 around the mouth. The nearly circular lateral organs, one-fifth 

 as wide as the head, are most conspicuous when the animal is seen 

 from above or below ; they then appear like two oblique lateral 

 pockets opposite to or a little behind the middle of the pharynx. 

 There are no eye spots, but granules of brownish pigment, arranged 

 in radial lines, are abundant in the anterior half of the oesophagus, 



♦ Cobb, " The Differentiator," Sydney, 1889. 



