150 ON PERMIAN ENTOMOSTRACA FROM 



This is much more so than with the specimens found in the 

 upper limestone at Byers' Quarry. Owing to this circumstance, 

 we can recognise the peculiar deltoid shape, hingeraent, and the 

 dorsal and ventral overlap of the sub-genus Bairdia in many 

 specimens. A very characteristic suite of these has been selected 

 and figured by Mr. Kirkby, besides some of the Cytheridw, 

 which do not (to me) appear to belong to Bairdia. 



At first sight it appears easy to sort out several carapaces and 

 valves of the Bairdice specifically distinct one from another; but, 

 on arranging the specimens (or drawings of them) in series, ac- 

 cording to certain modifications of their marginal outline, it 

 becomes a far more difiicult task to satisfy one's self where any- 

 thing like specific boundaries occur between them. The sub- 

 deltoid Bairdice occur fossil in several formations, and recent in 

 existing seas, and they present many varieties but little different 

 one from another, as far as the valves can show. There may be 

 specific differences of structure in the organs of the animals pos- 

 sessing the several varieties of carapace ; but of this we can say 

 nothing, for we know but very little of the animals. Giving 

 the greatest possible value I can to the differences observable in 

 the carapaces of the Permian specimens, I should say that figs. 

 1 and 2 (pi. IX.), represent the normal form of the Bairdia 

 plebeia of Keuss — or at least that well-marked, sub-deltoid form 

 of the species which Dr. Reuss described. Figs. 10, 9, and 12, 

 represent a narrow and acute form of the same species. Figs. 

 11, 4, 5, 6, and 3, are from specimens which graduate from the 

 sub-deltoid to a sub-oblong form ; every grade of the change is 

 readily found. In these varieties the lateral contour varies 

 within certain limits as to the lenticularity of the outline; a 

 strong middle overlap of the ventral margin is nearly always 

 present. Fig. 14 represents a nearly parallelopipedal carapace, 

 in which the approach to rectangularity is carried so far that we 

 seem to have overstepped a specific limit ; but even its modifica- 

 tion of shape, and greater size, do not give me any great confi- 

 dence in regarding it as a distinct species. 



Until recently I was disposed to consider all the above varie- 

 ties as belonging to the carboniferous species Bairdia curta, 



