248 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 55. 



maximum enlargement of the alimentary canal in segment V; anal 

 opening dorsal; caudal sucker ventral; body distinctly depressed; 

 posterior segments wider and flatter than the anterior segments. 



Type-species — Xironoglton orcgonensis, new species. 



Key to the three species and the two subspecies that have been 

 placed in this genus. 



a\ A somewhat concave, glandular adhesive disk near each lateral margin of 

 the ventral surface of segments VIII and IX; segments I to III con- 

 stricted anteriorly and posteriorly so that the segmental junctions are 

 well defined ; dental formula 6-5. X. occidentaUs, new species. 

 a\ No conspicuous adhesive disks on the ventral surface of segments "VIII and 

 IX, although several small glands are usually present ; body segments I 

 to IV subsequal and well defined, increasing slightly in width caudad; 

 body segments V to VM abruptly expanded and flattened, much wider 

 than the first four body segments; margins of segments V to IX so flat- 

 tened that the segmental junctions are almost obliterated ; general outline 

 of the body and head flask-shaped. 

 6*. The two teeth of the longest pair in the upper jaw separated by two 

 teeth; if two long teeth are contiguous the outer one is usually the 

 longer. X. oregonensis, new species. 

 c\ Dental formula 5-4, varying from 4-4 to 6-5. X. o. oregonensis, new 



subspecies, 

 c'. Dental formula 6-6, varying from 6-5 to 7-6. X. o. pectinatus, new 

 subspecies. 

 &'. The two teeth of the longest pair in the upper jaw separated by but one 

 tooth ; if two long teeth are contiguous the inner one is the longer. 

 X. instabilius (Moore). 



XIRONOGITON OCCIDENTALIS, new species. 



Plate 12, fig. 2. 



Type.— C^t. No. 17639, U.S.N.M., body length, 4.5 mm.. Crab 

 Creek, Washington (John T. Nichols), U. S. Bureau of Fisheries, on 

 Astacus MaTnathensis Stimpson. 

 Paratype. — One specimen collected with the type. 

 Description. — Body somewhat elongate and distinctly depressed; 

 body segments I to VIII distinct, each slightly constricted at its 

 anterior and posterior ends giving the segmental junctions sharp 

 definition; segment IX greatly reduced so that the body appears to 

 be composed of but eight segments; segments expanding gradually 

 and regularly in width to segment VII, which is the widest body 

 segment ; segment VIII almost as wide as segment VII ; segments IX 

 and X so narrow and inconspicuous that the caudal sucker appears 

 to be inserted under the posterior half of segment VIII ; head large, 

 its anterior third defined by a groove; lips tAvo, each with a slight 

 median emargination, otherwise entire; a few short transparent 

 bristles on the margins of the lips; major pharyngeal diverticula 

 three, two dorsal and one ventral; the ventral diverticulum about 



