226 Prof. T. It. Jones and Dr. H. B. Holl on 



slightly convex, and bear superficial ridges, both circular and 

 longitudinal (such as Kirhbya fibula, fig. 9, and K. costata, 

 M'Coy, sp.) ; but all have some trace of a subcentral pit or 

 notch, and the group is therefore distinct. These new forms 

 were referred to in the Quart. Journ. Geol. Soc. vol. xxiii. 

 (1867) pp. 494, 523, and 559, as Moorea obesa and M. tenuis, 

 Jones, MS. (once with a misprint of obtusa for obesa). We 

 have now to notice another and still older * species of this 

 ancient family of Entomostraca, the members of which seem 

 (as far as the carapace is concerned) to present closely related 

 genera, linking together Leperditia, Primitia, Beyrichia, 

 Ktrkbya, and Moorea by various gradations and resemblances 

 in the structure of the valves. Knowing that existing Ento- 

 mostraca, with mutually similar carapaces differ among them- 

 selves as to essential limb-characters, we feel more and more 

 inclined to lay stress on differences in the features of the fossil 

 valves, and to keep distinct all well-marked forms remaining 

 from among the almost lost tribes of these little primaeval 

 Crustacea. 



Moorea silurica, sp. nov. PI. XV. figs. 8 «, 8 b. 



Carapace-valves subovate, one-third longer than high, 

 slightly convex, polished, but coarsely punctate, and bearing 

 a raised marginal rim. Dorsal edge straight ; dorsal corners 

 rounded. Ventral border presenting a nearly true segment of 

 a circle. Ends somewhat obliquely rounded, nearly equal in 

 outline. A stout elevated ridge runs along nearly the whole 

 margin of the valve. It may be said to begin on the hinder 

 edge, which is depressed, but strongly lipped by the marginal 

 rim standing out sharply backwards ; it thickens on the ven- 

 tral border, is very thin anteriorly, and rises high along the 

 dorsal region, until it turns suddenly downwards, to lose itself 

 in the general surface of the posterior third of the valve. 



In some individuals the raised rim above the ventral border 

 is but faintly marked ; in others the marginal rim is strongly 

 developed, both above and below, and almost meets behind. 



Only single valves have been met with ; but in its ventral 

 aspect the carapace was probably somewhat like a compressed 

 orange-pip, partly split at one end. The greatest convexity 

 is about the centre of the valves. 



The ventral border of the figured valve (fig. 8 b, probably 

 the right and overlapping valve) is flattened suddenly by the 



* I do not fall in with Mr. Charles Moore's belief that the Entomos- 

 traca above referred to are of Liassie age, and were deposited in the fis- 

 sures from the sea of that period, but rather believe them to have been 

 derived by degradation from the fissured limestone. — T. R. J. 



