560 PROCEEDINGS OF THE NATIONAL MUSEUM. 



Lecane elegans. 

 Lecane hornemanni. 

 Lecane papunna. 

 Monostyla bulla. 

 Monostyla closterocerca. 

 Monostyla cornuta. 

 Monostyla decipiens. 

 Monostyla hamata. 

 Monostyla lunaris. 

 Monostyla obtusa. 

 Monostyla pyriformis. 

 Monostyla quadridentata. 

 Monostyla rugosa. 



Monostyla virga. 

 Lepadella cyrtopiis. 

 Lepadella triptera. 

 Colurella bicuspidata. 

 Colurella uncinata. 

 Trichocerca pusilla. 

 Trichocerca rattus. 

 Diurella stylata. 

 Diurella tigris. 

 Diurella voluta. 

 Polyarthra trigla. 

 Pedalia mira. 

 Collotheca polyphema. 



Of the 109 species and distinct varieties recorded, 46 are common 

 to the Atlantic and Pacific slopes, 55 occur only on the Atlantic, 

 and 8 only on the Pacific slope. This group of 63 species includes 

 but 20 occurring in sufficient numbers to be termed common — 16 

 Atlantic and 4 Pacific species. It is questionable whether these fig- 

 ures demonstrate anything beyond the already known lack of suit- 

 able environments on the Pacific slope. 



The closely related genera Lecane and Monostyla exhibit some inter- 

 esting anomalies of distribution. Of the 13 species of Monostyla, 

 12 are common to the Atlantic and Pacific slopes; Lecane is repre- 

 sented by 22 species, only 7 of which occur in both watersheds. 



Four species of these two genera, Lecane amorpha, L. fiexilis, L. 

 tenuiseta, and Monostyla hifurca, to which may be added a fifth, 

 Monostyla ( = Diarthra) monostyla, found by Daday in Paraguay, 

 occur on the Isthmus in ponds, while here at Washington they are 

 found only in Sphagnum. This change of habitat is without any 

 obvious explanation; the reverse would have been more intelligible. 

 If only one species had been involved it might have been passed over, 

 but when the same peculiar environmental relation is found to apply 

 to five species, there must be some definite cause for it, whatever that 

 may be. 



The fauna of Gatun Lake at the time the collections were made 

 was practically a normal pond fauna, as demonstrated by comparison 

 with similar locahties on the Isthmus. Rio Trinidad, in the region 

 represented, was virtually stagnant. It is surprising to find so few 

 species common to these two connected bodies of water. The lake 

 fauna would be expected to be transplanted from Rio Trinidad; evi- 

 dently this did not occur, and its origin must have been in the small 

 ponds which existed in the territory before the construction of the 

 lake and were gradually engulfed. 



If the Isthmian list is compared with Mun*ay's of the collections 

 made by him in Chile, Argentina, and Brazil, a sufficiently close agree- 

 ment wiU be found to warrant the conclusion that the rotatorian 



