evaporation and infiltration, and were now crowded to excess, prin- 

 cipally with the Apus, great numbers of which were dying upon the 

 surrounding mud, whence the water had receded. This species is 

 distinguished from the productus of Bosc and Montagui of Leach, 

 by not having the dorsal carina prolonged in a point behind ; and 

 from cancriformis by the greater proportional width of the thorax, 

 and more obtuse emargination behind. The length of the thorax along 

 the middle is three-tenths of an inch and its greatest breadth some- 

 what more. It may be named Apus obtusus." — Note 7. p. 336. 



5. Apus Domingensis, Baird, sp. nov. (Tab. XXII. fig. 1). Cly- 

 peo corporis dimidiam partem tegente, rotundo, tenui, cornea ; 

 ramo externa pedum primi paris corpus ceqaante. 



Long, toti corporis 1 poll. ; lat. clypei f poll. 



Hah. In Insula St. Domingo, India Occidentali. Collegit M. Salle. 

 Museum Britannicum. 



Though a native of the West Indies, this species may be easily 

 distinguished from A. Guildingii by its round-shaped carapace of a 

 homy colour covering half the body of the animal, and its external 

 branch of the first pair of feet only the length of the body, while in 

 A. Gnildingii it exceeds the whole body and caudal filaments in- 

 cluded. The carina dovni the centre of the carapace, and the fork 

 which it takes at the anterior extremity where the division into ce- 

 phalic and thoracic portions takes place, are marked throughout 

 their length with a deep brown colour, as are also the short stout 

 spines on the abdominal portion of the body. These are straight, 

 not hooked as in some of the other species. The caudal filaments 

 are nearly the length of the body, and are covered with very nume- 

 rous, extremely short setae. The oviferous feet are present in all the 

 specimens I have examined, but none contain any ova. 



Genus Lepidurus, Leach. 



Clypeus corneo-coriaceus. Corpus molle, cylindricum. Segmentum 

 caudale lamina producta instructum. Pedum primi paris ap- 

 pendices, aut rami, brevissimi. 



In the genus Lepidurus the tail-segment of the body, which in 

 Apus is rounded, is furnished with a flap or plate of considerable 

 size extending to some distance between the long setse or filaments. 

 The first pair of feet, compared with those of Apus, are extremely 

 short and comparatively inconspicuous. These two characters at once 

 distinguish the genus, of which only three species have as yet been de- 

 scribed. In other respects it resembles perfectly the genus Apus. 



Schseffer is the first author who has distinctly described any spe- 

 cies belonging to the genus Lepidurus. Linnaeus's description of 

 the " Monoculus cauda biseta" in the first edition of the 'Fauna 

 Suecica,' will apply to either the Apus or Lepidurus. He quotes 

 Frisch's figure, and states, as I have mentioned above, that he had 

 seen a specimen in London. We might conclude from this that it 

 was the Apus cancnformis he had in view ; but in the second edition 

 of the 'Fauna' (1761), he distinctly mentions, in his brief descrip- 



