176 Messrs. Jones and Kirkby on the 



stances we have seen them numerous enough to be the pre- 

 vailing fossils, and sometimes in such numbers as to be 

 gregarious. K. spiralis, J. & K., offers the most notable 

 example of this. In a thin, impure limestone of the Calcife- 

 rous Sandstones, near Pittenweem, Fifeshire, it is found in 

 great numbers ; and, again, just as abundantly in a local 

 intercalation of shale, in the Scar Limestone, at Meathop, 

 Westmoreland. K. costata also appears to be about as plen- 

 tiful in a Lower-Carboniferous deposit at Plashetts, North- 

 umberland, if we may judge from the numbers of specimens 

 that occur in a washing sent to us from that locality by our 

 friend Mr. James Bennie. These instances, however, are 

 exceptional in our experience. 



The generic characters of Kirkbya were noticed in a former 

 paper by one of us, as before mentioned (p. 175). They may 

 be restated here concisely as follows : — 



Carapace oblong, subovate, or ark-shaped ; flatly convex 

 or compressed. Valves usually thick-shelled, and generally 

 higher behind than before ; impressed with a subcentral pit, 

 and ornamented with longitudinal or concentric wrinkles, 

 riblets, ribs, or ridges, and often with surface-reticulation ; 

 the dorsal border is always straight ; the ventral border is 

 nearly straight, or slightly convex in its middle third, and 

 boldly curved at the ends ; and the extremities are more or 

 less rounded, though somewhat angulateat their junction with 

 the dorsal border; one end is generally more obliquely rounded 

 than the other. The hingement is simple. The ventral edge 

 of the dextral valve slightly overlaps that of the other. 



The subcentral pit is sometimes above and sometimes below 

 the median line of the valve, and it varies greatly in relative 

 size ; sometimes it is obsolete. In its typical form it is oval 

 or nearly round ; but in some species and varieties it becomes 

 irregular in outline, and passes, by gradations, almost into the 

 ordinary sulcus of Beyrichia. 



In the Ann. & Mag. Nat. Hist. ser. 4, vol. iii. 1869, p. 225 

 a list of the then-known Kirkbyw was given, and it was there 

 remarked by one of us, concerning the so-called varieties, that 

 11 some, if not all, of these may be distinct species ; for the 

 soft parts may have varied more than the carapace." The 

 probability of this being really the case grows upon us ? and 

 we now regard the " varieties " glypta, Rcesslerij Schrenhii r 

 sticta ( = Iiichteriana*) , and grapta as species. (See also 

 Ann. & Mag. Nat. Hist, for Nov. 1884, p. 340, footnote.) 



* This is the same as the " K. Rcessleri " described and figured by 

 Dr. R. Richter, Zeitschr. deutsch. geol. Ges. vol. vii. 1855, p. 528, pi. 20, 

 Iii)'*. 1-5. 



