228 BULLETIN 201, UNITED STATES NATIONAL MUSEUM 



Telson (fig. 97, h) short, about two-thirds the length of the sixth 

 somite of the pleon and only slightly more than half as long as the 

 inner uropod, iy 2 times as long as broad at the base, apex only two- 

 thirds the width at the base with a shallow broad and angular cleft, 

 lateral angles of the apex armed with a single spine, margins of the 

 cleft armed with seven spines on each side of the central point, these 

 spines shorter than those at the apical angles, lateral margins of the 

 telson armed with five spines on the proximal two-thirds of their 

 length, the distal third of the margins smooth and without spines. 

 Inner uropod 1% as long as the telson, without spines on its inner 

 margin. 



Outer uropod 2% times as long as the telson and one-third longer 

 than the inner uropod. 



Fourth pleopod of the male (fig. 97, g) with the endopod of normal 

 structure except that the side lobe is much shorter than in D. bahiren- 

 sis, exopod of one joint terminating in a very long stout plumose spine, 

 which is longer than the joint, three short spinules at the angle at 

 the base of the long spine. 



Length of an adult male 5 mm. 



Type lot. — Two males, 3 immature females, U.S.N.M. No. 81270, 

 from Paramaribo, Dutch Guiana. 



Occurrence. — South America: Paramaribo, Dutch Guiana, from a 

 ditch in the botanical gardens, fresh water, A. Reyne collector, 2 males, 

 3 females (type lot). 



Distribution. — The occurrence of the genus Diamysis on the Amer- 

 ican continent raises a point of geographical interest. The other three 

 species of the genus are inhabitants of the Mediterranean basin. 

 D. bahirensis is rather widely distributed along the shores of the Medi- 

 terranean and Black Seas, in water of low salinity. D. mecznikowi is 

 characteristic of the fresh waters draining into the Black Sea, and 

 D. pusilla is a Caspian Sea species. It is therefore of special interest 

 to find a species on the American continent in fresh-water conditions 

 similar to those of D. mecznikowi. It seems possible to explain this 

 interesting distribution of Diamysis by regarding D. americana on the 

 one hand and the Mediterranean species on the other hand as relicts 

 of the fauna of the old Tethys sea of early Tertiary times, in which 

 the genus was probably widely distributed. With the changes that 

 led to the present constitution of the Atlantic Ocean and the Mediter- 

 ranean basin the genus Diamysis gradually retreated on the one hand 

 into the Mediterranean and on the other hand into the Caribbean Sea, 

 penetrated into fresh water, and became the present-day Mediter- 

 ranean and American species. The American species can therefore 

 be seen to have a special interest for students of geographical distri- 

 bution. 



