from the Fossiliferous Limestone of Durham. 323 



subgeneric characters, which had scarcely been done by M'Coy*. 

 I attach to it a similar value in these remarks. 



Dr. Baird has placed it, as modified by Jones, among his 

 synonyms of Cythere. He states that no anatomical difference 

 exists in the animal of those recent species referred to it by 

 Jones, from the animal of Cythere properf. This is certainly a 

 strong point in support of its near affinity with Cythere; but 

 still, with its marked peculiarity of carapace, I think it may be 

 fairly entitled to a distinctive term to distinguish it from the 

 typical forms of Cythere. At least, whether it really be a natural 

 subgenus or not — supposing such to exist — it is a very convenient 

 group in palaeontology, as it serves to mark a number of fossil 

 Entomostraca of peculiar character, which have repeatedly held 

 no unimportant position in the economy of ancient faunas. 



The subgeneric characters of Bairdia have already been given 

 by Mr. Jones. The Permian species add little that is new to 

 them, though I will recapitulate the generic features of these 

 forms. They are all smooth. The general form of the majority 

 is somewhat triangular or subpentagonal, though species of a 

 different 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 in one or two instances 

 it is subangulated ; it is also much wider than the former. The 

 ventral margin is always the straightest, though often somewhat 

 sinuated. The dorsal margin is more or less convex centrally, 

 and generally sinuated towards the posterior extremity. In lateral 

 contour J they are lenticular, variously modified, being sometimes 

 almost exactly lenticular, at others, by the extreme position of 

 greatest diameter, subcuneiform. Perhaps the most important 

 generic character of Bairdia is its hingement, which is a lapping 

 of the left valve over the right along the dorsal margin, and 

 which was first noticed by Mr. Jones. In some Permian species 

 the folding of the valves is great (B. plebeia and B. ventricosa 

 may be cited as examples) ; 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 Permian species. A central 

 third of the ventral margin of the left valve overlaps an equiva- 

 lent portion of the right, giving the contact-line of the ventral 

 margins a sinuated appearance. The mode of its overlapping is 

 this : — from each extremity, until approaching the central por- 



* Entomostraca of the Cretaceous Formation of England, p. 22. 



t Natural History of the British Entomostraca, p. 163. 



X The term " lateral contour " is used for the outline which is seen by 

 viewing the carapace ventrally. " Ventral aspect " is used in similar in- 

 stances 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 former 

 better expresses the meaning attached to it. 



