Mysidacea, Tanaidacea and Iso])oda. 
409 
Telson short, apex convex, not split, armed with a comb of spines between 
two stronger lateral spines, margins with spines along their entire length. 
With the aid of Zimmer's key (1915) to the genera of the tribe Mysini, it is 
found that this genus falls into group II D and has its nearest ally in the arctic genus 
Stilomysis. It is to be distinguished from this genus mainly by the much different 
form of the telson, the absence of a row of spines on the inner margin of the uropods, 
the presence of lobes on the internal margin of the third and fourth joints of the 
endopods of the first thoracic limbs and by its very much smaller size. 
Superficially and especially in the females, it resembles the genus Potamomysis, 
as recently redefined by me, and can only be distinguished by a close examination of 
the shape of the telson. But whereas in Nanomysis, the third pleopod of the male 
has a well developed outer ramus, in Potamomysis it is a single- jointed plate as in 
the female. 
Nanomysis siamensis, sp. nov. 
(PI. XV, figs. 7-10.) 
Lucalily : — All the specimens in the collection were captured in the Tale Sap, 
Siam, in January, 1916, at the following stations: — 
St. 5. \ mi. E.N.E. of the mouth of the Patalung River, 2 metres, 13. i. 16, 
many, fragmentary. -f§-- (Fresh water.) 
St. 8. ^ mi. off shore a httle south of the mouth of the Patalung River, 2| 
metres, 14. i. 16, about fifty specimens. -f§^- (Fresh water.) Types. 
St. 10. Koh Si Hah, 17. i. 16, eight specimens, (Fresh water.) 
St. 25. Narrow channel opposite Ban Lem Chak, near Singgora, 6\ metres, 25. 
i. 16, one female, 4 mm., -f|^. (Sp. grav. 1-00425.) 
Description :— Carapace produced in front only slightly in the form of a rostral 
projection with a pointed apex ; the external portion of the frontal margin of the 
carapace on each side behind the eyes armed with a series of small spinules, those on 
the extreme outside the largest, the series gradually decreasing in size towards the 
centre ; pleon with the first and fourth segments shortest and equal in size, second 
and third slightly longer and equal, fifth segment slightly longer than the fourth, 
sixth segment i;^ times as long as the fifth, telson (PI. XV, fig. 8) only f of the length 
of the last segment of the pleon, about as long as broad at the ba.se, not cleft, lateral 
margins armed along their entire length with about 10 small spines with an addition- 
al larger spine at the outside corners of the apex, latter slightly convex, half as 
broad as the base of the telson, and armed with a comb of twelve .spines between the 
larger spines at each corner ; inner uropod two and a half times as long as the telson, 
without spines on its lower inner margin ; outer uropod only slightly longer than 
the inner, about longer. 
Second joint of the antennular peduncle very short, first and third joints about 
equal in size, male appendage well developed and densely hirsute, inner flagellum 
not much more than one- third, certainlj^ less than one-half of the length of the outer 
one and much more slender. 
