56 Prof. T. E. Jones and Dr. H. B. HoU on 



As one valve does not overlap the other in this Entomos- 

 tracan, it is not a Leperditia ; and the absence of both eye- 

 spot and muscle-spot also distinguishes it from that form and 

 the allied IsocMlina. The acutely elliptical depression of 

 the dorsal margins and the ventral rims remind us of similar 

 features in Primitia cristata^ P. umhilicata^ and P. tersa (Ann. 

 Nat. Hist. I. c. pi. 13. figs. 1-3) ; and a ventral rim is charac- 

 teristic also of other Primiticej whether the median pit or fur- 

 row is present or not. 



P. Maccoyii is very abundant in the limestone of the Chair 

 of Kildare. 



Several years since^ Mr. Salter intimated that this fossil 

 could not be the same as Hisinger's Cythere phaseolus. The 

 latter, we know, is a Leperditia closely related to (or the young 

 of) L. BaltMca) and, though figm'ed roughly in Hisinger's 

 ^ Lethaea Suecica ' (pi. 1. fig. 1), with a mere ovate outline (as, 

 indeed, L. BaltMca also was at first), it is really Leperditioid 

 in shape, and has other characters of the genus. 



An individual P. Maccoyii is present in one of the specimens 

 of Bala-Caradoc limestone from Aldeans*, on the Stincher 

 (or Stinchar) E-iver, in Ayrshiref, preserved in theWoodwardian 

 Museum at Cambridge, and, indeed, appears to have been no- 

 ticed, though not recognized, by Prof. M'Coy (see Ann. Nat. 

 Hist. ser. 2. vol. viii. p. 387 ; and further on, p. 60). 



In the equivalent limestone of Keisley, in Westmoreland, 

 which has a close afiinity, both in fossils and mineral charac- 

 ter, with that of the Chair of Kildare, P. Maccoyii has been 

 discovered by Prof. Harkness (see his account of the Lower 

 Silurian Rocks of Westmoreland, Quart. Joum. Geol. Soc. 

 vol. xxi. pp. 243 &c.). 



2. Primitia 8ancti-Patriciij n. sp. PL VII. figs. 4 a, 4 Z>. 



Carapace smooth, almost semicircular in outline, convex in 

 the middle and nearly equally compressed towards the margin 

 all round ; back very slightly arched, rounded at the end of 

 the hinge-line ; one extremity rather more broadly curved than 

 the other ; ventral margin fully convex, and bordered (espe- 

 cially posteriorly) with a faint rim where the edge of the valve 

 turns inward. Dorsal profile acute-oval. 



Rather more semicircular than P. ohsoleta^ this Irish species 

 differs from it also in having less of the marginal rim and no 

 sulcus, and in being more oval than ovate in the profile of the 

 closed valves. Indeed it seems to be intermediate between P. 



* Also written Aldens and Aldons. 



t Rep. Brit. Assoc, for 1850, Trans. Sect. p. 107 ; Quart. Joum. Geol. 

 Soc. yol. viii. (1851) pp. 139 &c. j and ^Siluria/ 3rd edit. 1867, p. 156. 



