154 CARBONIFEROUS LAMELL1BRANCHIATA. 



Interior. — The anterior adductor scar is moderately large, shallow, not 

 marginal ; the posterior is situated "within the posterior slope, and is elongate and 

 remote from the posterior margin. The hinge-plate has two or three oblique short 

 teeth anteriorly, and four elongate posterior lateral teeth, arranged so that the 

 upper one is the longest, and those below gradually shorter ; all terminating 

 behind at the same place, which is not near the posterior margin. The pallial 

 sinus is entire and remote from the border. 



The exterior surface is ornamented with fine lines of growth, which become 

 more widely separated as they occupy a position nearer the inferior edge of the 

 valve ; under the microscope these are decussated by very obscure radiating ribs, 

 less visible at the anterior end. 



Dimensions. — Fig. 17, PI. XII, measures — 



Antero-posteriorly .... 23*5 mm. 

 Dorso-ventrally . . . .10 mm. 



Elevation of valve . . . .3 mm. 



Localities. — England : the Carboniferous Limestone of Castleton and Thorpe 

 Cloud, Derbyshire ; Hill Bolton, Yorkshire. Middle White Limestone, Flintshire. 

 Ireland : Arenaceous Limestone, Townparks, Killeshandra. 



Observations. — The type specimen of this species, preserved in the Griffith 

 Collection in the Dublin Science and Art Museum, under the title Modiola concinna, 

 is not a very perfect specimen (PI. XII, fig. 16), still there is enough of it to 

 demonstrate that it is a member of the Area family. The peculiar shape of the 

 posterior end and the ornamentation are very characteristic, and serve to distin- 

 guish the species from all others. Owing, doubtless, to the shell being described 

 under the generic name Modiola, the species seems to have been identified by no 

 authors with the exception of R. Etheridge, who inserted the name in his cata- 

 logue under the original generic name. 



I have been fortunate enough to obtain from the limestone of Hill Bolton 

 (which, from its situation, most probably is composed of the uppermost beds of 

 that deposit) a specimen, PI. XII, fig. 22 a, which gives most perfect details of the 

 interior from a cast. It possesses the peculiar shape of posterior end so charac- 

 teristic of the species, and has both ends of the obverse of the hinge-plate preserved, 

 which is shown to be true to the generic type. The muscle-scars are also shown. 

 The peculiarity in the cast itself is the absence of any concentric or radiating 

 markings, which in most species are present on the interior of the valves, or at 

 least on the dorsal slope. 



