REPORT ON THE OSTRACODA. 49 



branchial plate. The genera Pontocijpris and Macrocyiyris also show intermediate 

 characters in the structure of the second pair of maxillse. 



Of the twenty-three species oi Bairdia noticed in this monograph, one only, Bairdia 

 villosa, was taken alive. All the rest are represented merely by dead shells. In this 

 condition it will be seen that the task of specific identification becomes very difficult, 

 the only available characters being those of the shell, which in this genus does not show 

 any very marked specific differences of surface-ornament ; the shape and proportions of 

 the shell thus become the only available diagnostic marks, and it is very probable that 

 the further investigation of larger numbers of specimens, and above all, of living 

 animals, may very much modify our view as to the validity of some of the characters 

 here adopted as specific marks, but which may prove to be dependent upon sex or stages 

 of growth. It need scarcely be said that many of the fossil forms described by authors 

 under the generic name Bairdia, must of necessity lie transferred to other genera, and 

 the same observation holds good as regards Cythere, Cytheridea, Cypridina, and other 

 names in use by palaeontologists before our anatomical knowledge of the group had been 

 much elaborated. But as important anatomical differences are constantly coincident 

 with weU-marked shell characters,^ it is possible in most cases to refer even fossil species 

 to their proper generic position, the difficulty being, indeed, no greater than constantly 

 occurs with recent dredged specimens, in which the animal contents of the shell have 

 entirely disappeared. 



1. Bairdia fusca, G. S. Brady (PI. VII. fig. 2, a-d). 



Bairdia fused, Brady, Trans. Zool. Soo. London, 1865, vol. v. p. 364, pi. Ivii. iig. 9, a-cl. 



Carapace as seen from the side subtriangular, greatest height situated in the middle, 

 and equal to about two-thirds of the length ; extremities rounded, but more broadly 

 in front than behind ; ventral margin nearly straight ; dorsal very boldly arched ; seen 

 from above, the outline is compressed, ovate, subacuminate behind, and rather more 

 obtusely pointed in front; greatest width in the middle, scarcely equal to half the length ; 

 end view ovate, height much exceeding the width. SheU-surface smooth (slightly hairy 

 when recent), and covered with closely set minute punctations. Length, l-25th of an 

 inch (1 mm.). 



A few specimens of Bairdia fusca occurred in a dredging made in very shallow 

 water (2 to 10 fathoms) at Port Jackson, Australia. The species was described by 

 myself in 1865, from Australian specimens, which agree entirely with those brought 

 home by the Challenger, except that these last, being only dead shells, are pale in colour 

 and have lost all their hairs. 



1 So far as British Post-Tertiary species are concerned, these characters have been tabulated in Messrs Brady, 

 Crosskey, and Robertson's Monograph of the Post-Tertiary Entomostraca, issued by the Palwontographical Society. 

 (zool. chall. EXP. — PART III. — 1880.) C 7 



