3o4 SUPPLEMENT. 



3. Male with enormously developed costa nimius sp. n. 



Male with scarcely enlarged costa 4 



4. Lamellae of hypopygium small, triangular triangularis, sp. n. 



Lamellae of hypopygium larger, quadrangular [superbus, Aid.] 



[l. Polymedon flabellifer. 



Polymedon flabellifer, O. Sack. Western Diptera, p. 317 \ 

 Edb. Noeth Amekica, San Francisco \ Idaho. 



This species occurs in the neighbourhood of San Francisco, on rocks, in swift 

 streams. I collected specimens in a similar place at Juliaetta, Idaho, on September 

 15th, 1898. P. flabellifer must be regarded as the highest member of the known 

 series, since it has dropped the deformity of the middle tarsi, characteristic of the 

 males of all the other species, and has developed an ornamental silver stripe down the 

 dorsum of the thorax, involving the complete suppression of the acrostichal bristles. 

 Viewed from before, this stripe seems almost continuous with the silver of the face 

 and front, though less deep. These peculiarities, of course, occur only in the male. 



A very easily recognized species.] 



2. Polymedon argentatus, sp. n. (Tab. VI. %. 7, wing of e .) 



5 . Black, with a slight greenish reflection ; the tibiae, tips of coxae and femora, and the halteres, yellow. 

 Face rather wide, gently bent above the middle, pure silver in its whole length, projecting below the level 

 of the eyes a little less than its own width ; antennae of ordinary size and structure, wholly black ; front 

 silvery, except the ocellar tubercle; orbital cilia white, except a few of the uppermost. Thorax dark 

 green, but little metallic ; on the sides, in a certain light, with a silvery reflection from the humerus to the 

 root of the wing ; pleurae black, with a faint white dust when viewed from before. Teguhe yellowish, 

 the margin infuscated ; the cilia black, at the ends whitish. Abdomen blackish-green, on the sides with 

 a little white dust behind the incisures ; hypopygium black, lightly dusted, moderate in size, the triangular 

 lamellae sordid whitish near the base, black on the margin. Base of front and middle tarsi slightly 

 yellowish ; middle tarsi with a bend between the second and third joints, but without any great modifi- 

 cation of structure ; hind tibiae a little infuscated at the tip ; hind metatarsi shorter than the following 

 joint. Wings with a considerable and uniform infuscation ; the costa a little enlarged in its first section, 

 more so at the junction of the first vein, from there gradually tapering to the normal size, which is 

 attained over halfway to the second vein ; fourth vein with a very gentle curvature. 



Length 5 millim. 



Hab. Mexico, Cuernavaca in Morelos (H. II. Smith). 

 One male. 



3. Polymedon nimius, sp. n. (Tab. VI. figg. 8, 8 a-c, e *; 9, $ .) 



A large, elongate, slender species ; the legs and antennae almost entirely yellow. 



c? . Face wholly silvery, spatulate, extending below the level of the eyes about twice its own width; antennse 

 rather slender, yellow, the upper edge and the tip of the third joint black ; orbital cilia white, except a few 

 of the upper ones. Dorsum of thorax dark metallic green, with an indistinct violet median stripe ; pleurse 



* Incorrectly marked $ on the Plate. 



