ON BOSMINIDJi:, MACKOTHKICTDiE, ETC. 379 



such cases, by reserving the name of Miiller for that species out 

 of several closely allied forms which is most common, and which 

 most subsequent authors have regarded as the type. 



Many of the figures and descriptions of "Lynceus quadrangu- 

 laris," given by authors, are insufficient to determine with accu- 

 racy whether the present form, or one of its allies, is intended. 

 Leydig gives capital figures of the species, and describes it as new 

 under the name of Lynceus affinis ; while he in error describes 

 and figures as Lynceus quadrangularis a widely different species, 

 namely, the Lynceus acanthocercoides of Fischer and of this paper. 



6. Lynceus costattjs, (G. 0. Sars). PL XYIII, fig. 2; and 

 PI. XXI, fig. 7. 



1862. Alona costata, G. 0. Sars. Om de i Omegnen af Chris- 

 tiania forkommende Cladocerer. Andet Eidrag, p. 88. 



Carapace quadrangular, short, and very broad ; dorsal margin 

 well arched ; posterior margin truncate, but its angles above and 

 below well rounded off ; no teeth at the infero-posteal angle ; 

 ventral margin straight, fringed with cilia ; surface longitudinally 

 striate. Head nearly erect, large, with a blunt, hood-formed 

 rostrum, not projecting beyond the ventral margin of the cara- 

 pace ; anterior antennae entirely beneath the hood ; posterior 

 antennae short, one branch bearing three, the other four setae. 

 Eye-spot scarcely half the size of the eye, situated midway be- 

 tween the eye and the extremity of the rostrum. Abdomen very 

 short, gradually narrower towards the distal extremity, which is 

 truncate ; superior margin straight throughout and having a dis- 

 tinct posteal angle, which is produced and not rounded off ; armed 

 on the edge with from ten to fourteen spines, which gradually 

 increase in size posteriorly ; a small process at an obtuse angle, 

 which is situated, as in L. quadrangularis, midway between the 

 uppermost of the caudal spines and the origin of the abdominal 

 seta? ; terminal claws large, gently curved, and provided with a 

 small secondary spinous process springing from near the base, but 

 not one-fifth of the length of the claw itself, and of about equal 

 size with one of the large abdominal spines. Length, xg-th of an 

 inch. 



