170 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.41. 



KEY TO LETTERING. 



ADA Anterior dorsal area. L S C Lateral spinal cleft. 



AIR Anterior interspinal ridge. MR Marginal ridge. 



BO Body outline. . MVR Margino-valval ridge. 



C R Cervical ridge. C Ocular cell. 



D R Dorsal ridge. R Occipital ridge. 



F A Facial area. PDA Posterior dorsal area. 

 F R Facial ridge. P I R Posterior interspinal ridge. 



H Horn. S S R Superior spinal ridge. 

 I C Interspinal cleft. S R Submarginal ridge. 



ISR Inferior spinal ridge. S VR Superior valval ridge. 

 I V R Inferior valval ridge. 



Plate 18. 



Fig. 1. Chydorus bicornutus, female from Sebago Lake, Maine, July, 1908. Anterior 

 view. Lettering as on Plate 17. X127. 



2. Do. Antenna. X233. 



3. Do. Antennule. X400. 



4. Do. Lower part of head, lateral view. X233. 



5. Do. Labrum, anterior view. X233. 



6. Do. Maxilla. X400. 



Plate 19. 



Fig. 1. Chydorus bicornutus, female from Sebago Lake, Maine, July, 1908. First 

 leg. X400. 



2. Do. Second leg. X400. 



3. Do. Third leg. X400. 



4. Do. Fourth leg. X400. 



5. Do. Fifth leg. X400. 



6. Do. Postabdomen. X233. 



Lettering: d, exopodite; e, e' , e", endopodite and its branches; mx, maxillary 

 process. 



