146 ONYX AND DIPELTIS, 



An excellent way is to make the whole apparatus represented in 

 Fig. 3 so small that it can be readily introduced into the object box 

 of a differentiator. When the object returns from the differentiator 

 the compressorium is carefully removed and the object will be 

 found not to adhere to the covers, providing they were originally 

 clean. It would be difficult to exceed the perfection of objects thus 

 treated. The covers should not lie horizontal in the differentiator, 

 otherwise the time occupied in treatment will be lengthened owing 

 to the difficulty with which the fluids will enter the space between 



them. 



II. 



The New Genus Onyx. 



In the worms constituting the genus Onyx the structure of the 

 head and neck is very characteristic, but at the same time the 

 kinship with the genus Dorylaimus is at once evident. It will be 

 presently seen, however, that the two genera are very distinct 

 from each other. 



As one would expect from the foregoing remark the pharynx in 

 Onyx is armed with a spear. As in Dorylaimus, so here, the 

 spear is axial and attached to the dorsal side of the pharynx. 

 The uncertainty with regard to the length of the spear is how- 

 ever lessened in Onyx by the presence of a distinct pharyngeal 

 swelling or bulb, which is, as far as length is concerned, nearly 

 co-extensive with the spear. This pharyngeal bulb is an elongated- 

 ellipsoidal, muscular swelling, several times wider than the spear 

 which it encloses ; its function is by longitudinal contraction to 

 protrude the spear. This latter organ is stout and tipped with a 

 characteristic horny structure, from which the genus takes its 

 name. As seen under the microscope, this tip presents itself as 

 an inverted V-shaped, or more properly sagittate, body having an 

 opalescent appearance. It is not quite symmetrical, for the 

 ventral barb is uniformly slightly longer than the dorsal. I 

 hardly need remark that this description refers to the view usually 

 obtained, that is to say, the profile or lateral view, and that this 

 body is in reality a hollow cone. The ring so constantly present 

 in the throat of Dorylaimus is paralleled in Onyx by a three-fold 

 growth from the walls of the anterior part of the pharynx, whose 



