THE SHELL-LIMESTONE OF DURHAM. 131 



The variation of the posterior extremity is of most importance, 

 as in some cases the slight modification which it undergoes 

 causes it to assume the form of the anterior. 



One of the most peculiar characters of this species is the curi- 

 ous marginal rims which bound its free margins, and form so im- 

 portant a feature in its ventral aspect. I have never observed 

 more than two rims on each valve, except in one instance, which 

 was a perfect specimen, having three rims on the right valve, 

 with only two on the left. These rims very much remind one of 

 the exfoliative dilatations of the margins which are seen in some 

 species of Mollusca ; but when the mode of growth of the Ento- 

 mostraca is considered, such an idea is found to be untenable ; for 

 we must suppose that these animals, like their recent represen- 

 tatives, would increase in size by moulting, and not by marginal 

 increment. The youngest specimens possess the rims in minia- 

 ture; indeed, all stages of growth are characterised by them, 

 though the older individuals have them most produced. One of 

 my specimens (fig 11, pi. X.) appears to have several fine lines 

 between the outer and inner rims and running parallel to them. 

 The same portion of the valves are occasionally seen to be 

 delicately reticulated after the pattern of the rest of the surface. 

 (Fig. 5c, pi. X. A.) 



The central area of the valves is generally very much pro- 

 duced, but more so in some specimens than in others. Some- 

 times its connection with the marginal portion of the valves is 

 so abrupt as to cause it to appear like a great tubercle ; at others 

 it slopes more gradually towards the margin, and wears a less 

 gibbose aspect: this is particularly the case in young specimens. 

 The postero-dorsal region of this area is always the most pro- 

 minent portion of the valve ; and as the central portion of the 

 dorsal region is at times rather depressed, both it and the antero- 

 dorsal angle have there a gibbose appearance. Such specimens 

 assimilate to the IC {Cythere) Schrenkii^ of Keyserling, the 

 equivalent regions of which are extremely gibbose. As the 

 central area varies in prominence in different specimens, so do 

 specimens vary in width, and that very considerably. 



* " Reise durch die Tundren der Samojeden, 1854," p. 112, taf. 4, fig. oT. 



