324 Mr. J. W. Kirkby on Permian Entomostraca 



tion of the ventral margin, the extreme edges of each valve are 

 somewhat produced, and lie against each other in close juxta- 

 position, like the valves of a conchifer ; towards the centre, how- 

 ever, the edges become flattened and bent inwards, forming 

 slight horizontal ledges or flanges, that of the right valve being 

 rather smaller than the other which overlaps it. In no instance 

 does the whole margin of one valve overlap that of the other ; in 

 no species is more than a third overlapped, sometimes less. The 

 length of margin overlapped, and its width, are of use as specific 

 characters, as is also its position, which is not constant, being 

 sometimes nearer one extremity than the other. 



Although nearly all the species of Bairdia possess a contour 

 more or less resembling those species which may be considered 

 typical of the group, such as B. curta (Carboniferous), B. plebeia 

 (Permian), and B.subdeltoidea (Cretaceous), there are others which 

 show a great aberration in outline from these types, and, so far 

 as their general form is concerned, have a much nearer resem- 

 blance to Cythere than to Bairdia; their hingement, however, 

 being that of Bairdia, leaves no doubt of their true affinity. 

 Among the Permian species, B. reniformis, B. Berniciensis, and 

 B. Jonesiana are the most aberrant in this respect. 



1. Bairdia plebeia, Reuss. PI. X. figs. 1, 2, & 2 a. 



B. plebeia, Reuss, Jahresbericht der Wetterau. Gesell. 1851-1853, p. 67. 

 Syn. B. curta, M'Coy, Jones, Mon. Perm. Foss. of England, p. 61. tab. 18. 

 fig. 3. 



Length T ' y inch ; height -fa inch. 



Carapace sub trigonal, somewhat inflated centrally, surface 

 smooth. Dorsal margin prominently convex, sometimes rather 

 flatly convex ; posterior slope abrupt, slightly concave ; anterior 

 slope less abrupt, straight, or slightly concave. Ventral margin 

 straight centrally, more or less convex towards each extremity. 

 Anterior extremity rounded, and rather produced. Posterior 

 extremity acute, rostrated. Lateral contour regularly lenticular ; 

 greatest diameter (of lateral contour, or width of carapace) two- 

 thirds of the height. The flange, or overlapping portion of the 

 ventral margin of left valve, short, subcentral. Hinge with the 

 left dorsal margin overlapping the right extensively. 



Mr. R. Jones has identified a very similar form, found in the 

 upper members of the Durham Permians at Byers' Quarry and 

 in the fossiliferous limestone of Humbleton Hill, with the B. 

 curta of M'Coy. I think it very probable that the former is 

 identical with the present species, although the figure and de- 

 scription of Jones are somewhat different, — a difference, how- 

 ever, which is most likely due to the state of preservation in which 

 his specimens occurred, being casts. His figure represents the 



