from the Fossiliferous Limestone of Durham. 435 



valve, and which appears to overlap (?) a similar longitudinal 

 projection in the opposite valve. The outer surface of the valve 

 within the innermost rim is deeply channeled. The central area 

 of the valves is protuberant, rising abruptly from the channeled 

 depression just mentioned ; dorsally and towards the posterior 

 extremity this area is very prominent, or slightly gibbose. Sur- 

 face ornamented with irregularly-placed roundish pits, and with 

 slender longitudinal wrinkles which occasionally bifurcate and 

 merge into each other. Hinge with the dorsal margins united 

 by ligament (?) . Lateral contour ovate, with strongly produced 

 extremities. 



This remarkable species does not vary much in marginal out- 

 line. The posterior extremity is occasionally rather less pointed 

 than usual, and the anterior extremity differs a little in con- 

 vexity ; the ventral margin also has at times a tendency to be- 

 come sinuate. 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 

 curious marginal rims which bound its free margins and form 

 so important a feature in its ventral aspect. I have never ob- 

 served 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 exfoliative dilatations of the margins, such as are seen in 

 some species of Conchifera and Brachiopoda : but when the 

 Entomostracan mode of growth is considered, the idea is found 

 to be untenable; for we must suppose that these species, like 

 their recent representatives, would increase in size by moulting, 

 and not by marginal increment. The youngest specimens pos- 

 sess the same rims in miniature; indeed, all stages of growth 

 are characterized by them, though the older individuals have 

 them most produced. One very fine specimen (fig. 11) shows 

 several fine lines between the outer and inner rims, and running 

 parallel with them. I have not been able to prove satisfactorily 

 that the longitudinal projection on the internal surface of the 

 most ventral rim (see fig. 13) of one valve overlaps that of the 

 opposite, though, from the close union of the extreme edges of 

 these rims in some specimens, it may be inferred that it does. 

 The free margins fit close ; consequently the whole of the animal 

 must have been enveloped by the valves. 



The central area of the valves is generally very much produced, 

 but more so in some specimens than in others. Sometimes its 

 connexion 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 



