54 TWELFTH ANNUAL REPOET. 



jaws, which latter are the larger in the embryo, being exceedingly 

 large nerves. 



This key contains the majority of the genus, but falls short of 

 completeness. The following species are uncertain. W. Schman- 

 kewitsch described as new D. degenerata and D. rudis^ from salt 

 or brackish waters. These he regards as degenerate forms produced 

 by the inferior aeration of dense waters. The author does not 

 appear to recognize the modern distinctions of genera so that, not 

 having seen the work, even the generic position can not be defi- 

 nitely stated. His investigations seem to show that the proximity 

 of salt waters influence the form of the body, or, perhaps, that 

 there is a constant interchange between the sub-marine and fresh- 

 water species. Daphnia hrevicauda^ Chambers, is an incorrectly 

 figured and described Simocephalus. 



Key TO THE GrENUs Daphnia. 

 Section i. Pigment fleck present. 



a. Head short, equally rouucled. 



1. D. psi«ocea, Baird. 



B. Head not regularly rounded, more or less beaked. 



(a) Claws spiny. 



I. Abdomen broad, series of anal spines nearly equal, neither head nor back, 

 keeled. 



t A marked sinuosity in the posterior outline of post-abdomen. 



2. D. scheefferi, Baird. 

 D. ovata, Sars. 



D. pennata, Mueller. 

 tt No well marked depression. 



3. D. pulex, Mueller. 



4. D. schcedleri, Sars. 

 D. hastata, Sars. 

 I), obtusa, Kurz. 



II. Abdomen narrow, shell keeled somewhat dorsally. 



5. D. oninnehaha, sp. n. 



6. D. carinata, Sars, 



(b) Claws nearly or quite smooth. 

 I. Head not crested. 



7. D. longispina, Leydig. 



8. D. rosea, Sars- 



9. D. simiiis, CI aus 

 D. lacustris, Sars. 

 D. cavitrons, Sars. 



10. D. hyalina. Leydig. 



11. D. dubia, Herrick. 



