224 Prof. T. R. Jones and Dr. H. B. Holl on 



Nat. Field Club, vol. iv.) Since then, several other Kirkbyce 

 have been recognized in the Carboniferous formations. (See 

 Jones and Kirkby's papers in the Ann. Nat. Hist. ser. 3. 

 vol. xviii. pp. 42,43,45,49, and Transact. Geol. Soc. Glasgow, 

 vol. ii. pp. 216 &c.) 



In this genus the carapace- valves are compressed (flattish), 

 thick, oblong, impressed with a subcentral pit and raised into 

 ridges, some concentric with the margin, associated sometimes 

 with longitudinal riblets or wrinkles, and often accompanied 

 by a reticulate ornament. In shape the valves are subob- 

 long, usually higher behind than before ; the extremities more 

 or less rounded, but one often much more obliquely than the 

 other ; the dorsal border is straight, and its ends are subacute ; 

 the ventral border is nearly straight in its middle third, and 

 boldly curved at the ends ; the hinge is simple. The ventral 

 edge of the dextral valve overlaps slightly that of the other. 

 The subcentral pit or sulcus is sometimes above and sometimes 

 below the median line of the valve, and varies greatly in its 

 relative size. 



Kirkbya, having relationships with Beyrichia, Primitia, 

 Moorea, and Leperditia, is one of the Leperditiadce (see Ann. 

 Nat. Hist. 1856, ser. 2. vol. xvii. p. 99). To Leperditia it is 

 related through Beyrichia and Primitia. In general form, 

 hingement, ventral overlap, and even sometimes in a faint bi- 

 lobation of the surface, the valves of Kirkbya resemble those 

 of Beyrichia • but the double and sometimes threefold ventral 

 rims, and especially the subcentral pit and the longitudinal 

 riblets, distinguish them. The ventral ridges to some extent, 

 and the pit, have their analogues in Primitia ; but this genus 

 generally presents convex forms ; and when flattish, its valves, 

 though sunken in P. excavata, have not any costation. Moorea 

 presents flattish valves, marginally ridged, but without any 

 subcentral pit or dorsal furrow (see further on, p. 225). 



Kirkbya jilnda, sp. no v. PI. XV. figs. 9 a, 9 b. 



Valves oblong, with three of the borders more or less rounded ; 

 rather short, flat, bearing a well-marked marginal rim, which 

 in some specimens dies away on the upper border, where the 

 subcentral pit deeply notches the dorsal region somewhat to- 

 wards the anterior angle ; and in others it is strong on the dorsal 

 and dies away along the anterior border. A neatly defined 

 longitudinal ridge, slightly sigmoid in outline, and thickest at 

 its middle part, traverses the depressed surface of the valve 

 somewhat obliquely, within the marginal ridge, from near the 

 postero-dorsal angle to the middle of the anterior border, and 

 at its junction with this it bends a little downwards. Near 



