140 ON PERMIAN ENTOMOSTRACA FROM 



the carapace is finely pitted, and occasionally hirsute in the re- 

 cent state. The general form of the majority is somewhat trian- 

 gular or subpentagonal, though species or varieties having a still 

 more elongate contour occur. The posterior extremity of all is 

 more pointed than the anterior, and it is generally rostrated. 

 The anterior extremity is usually rounded, but sometimes sub- 

 angulated, and also much wider than the posterior extremity. 

 The ventral margin is always the straightest, though often some- 

 what sinuated. The dorsal margin is more or less convex cen- 

 trally, and generally sinuated towards the posterior extremity. 

 In lateral contour* they are lenticular, variously modified, being 

 sometimes almost exactly lenticular; at others, by the extreme 

 position of their greatest diameter, sub-cuneiform. Perhaps the 

 most important generic character oiBairdia is its hingement, which 

 is a lapping of the left valve over the right, along the dorsal-mar- 

 gin, and which was first noticed by Mr. Jones. In some Per- 

 mian varieties, the folding of the valves is relatively great ; and 

 in most cases the folding is much greater in the centre than near 

 the extremities. In general this character does not vary much 

 in the Permian species. A central third of the ventral margin 

 of the left valve overlaps an equivalent portion of the right, giv- 

 ing the contact-line of the ventral margins a sinuated appear- 

 ance. The mode of its overlapping is this: — From each ex- 

 tremity, until approaching the central portion of the ventral 

 margin, the extreme edges of each valve are somewhat produced, 

 and lie against each other in close juxtaposition, like the valves 

 of a conchifer ; towards the centre, however, 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 

 one-third overlapped, but sometimes less. The length of margin 

 overlapped, and its width, may occasionally be of use as sj^ecific 



* The term " lateral contour," is used for the outline which is seen by viewing the cara- 

 pace ventrally. " Ventral aspect," is used in similar instances by others ; but, as it is 

 chiefly the contour of the sides of each valve of the carapace that is referred to when using 

 the term, I think the foiTner better expresses the meaning attached to it. 



