LIMATULINA. 35 



Exterior. — The surface is ornamented with numerous concentric, rounded, low 

 ridges and grooves, not always equidistant. 



Dimensions. — PI. VI, fig. 1G, from Castleton, measures — 



Antero-posteriorly .... 25 mm. 

 Dorso-ventrally . . . .30 mm. 



From side to side . . . .12 mm. 



Locality. — The Carboniferous Limestone of Castleton, Derbyshire. 

 Observations. — De Koninck states that P. obliqua occurs very rarely in the 

 Carboniferous Limestone of Vise. It is not at all a common shell at Castleton, 

 which is the only locality at Avhich I have yet obtained the species, but several 

 specimens have been found here. The shell is much more gibbose and narrower from 

 side to side than /'. Becheri, and the concentric ridges are much closer and narrower. 

 The umbones do not seem to have been contiguous, one of the specimens showing 

 :i small area ; but no details of the nature of the hinge-plate have been observed. 



Family LIMID^E. 



Genus Limatulina, de Koninch, 188G. 



Limatulina, pars, de Koninck, 1885. Ann. Mus. Koy. d'Hist. Nat. Belg., torn, xi, 



p. 243. 



Generic Characters. — Shell small, very inequivalve, generally obliquely swollen, 

 triangularly ovate. The left valve gibbose, its umbo overlapping the right valve, 

 which is flat. Ears depressed, the posterior not being well defined. The anterior 

 ear in the right valve deeply slit for the byssus. The hinge-line straight, and the 

 hinge-plate consisting of a single transverse groove. Exterior surface ornamented 1 ir- 

 radiating stria? and ribs, which are generally close and numerous, and often rugose. 



Observations. — This genus was established by de Koninck for a series of inequi- 

 valve, /V, •/>/, -like shells in which the left valve is much larger than the right valve, 

 and the left umbo arches over and overlaps the small, almost obsolete umbo of the 

 right valve. De Koninck described six species, but I do not think L. selecta can 

 belong to this genus. M'Coy's Lima aJternata should certainly, I think, be placed 

 in the genus, while shells with well-defined posterior ears should be excluded. 



The genus Limatulina is closely related to Wumicrotis, but the latter has a 

 smooth shell ; though the inequality of the valves and the different characters of 

 the umbones are very similar to the condition in each genus. 



G 



