72 CARBONIFEROUS LAMELLIBRANCHIATA. 



near the dorsal margin. The pallial line is entire, remote from the margin, and 

 extends posteriorly for some little distance beyond the posterior adductor scar, its 

 upper limb returning forwards to terminate in the posterior inferior corner of the 

 scar. 



Exterior. — The shell is very thin, and is ornamented with irregular fine excentric 

 lines of growth oblique to the long axis of the shell, crowded and more conspicuous 

 in front, becoming more widely sejDarated and at the same time less distinct ag they 

 pass over the more tumid portion of the shell. Towards the lower margin and 

 behind, these are crossed by very fine radiating striae. Near the posterior extremity 

 of the valves the markings become almost obsolete. On the surface of casts the 

 markings on the interior of the shell show similar characters. 



Dimensions. — Fig. 1, PI. VIII, measures — 



Antero-posteriorly . . . .43 mm. 



Dorso-ventrally . . . .20 mm. 



Laterally . . . . .17 mm. 



Localities. — England : The Limestone of Lowick, Northumberland, and the 

 Middle Limestone of Wensleydale.^ Scotland : Carluke, in the Lingula Ironstone. 

 Ireland : Carboniferous Limestone of Killogunra, Killala. 



Observations. — This species was described by M'Coy, under the genus Litho- 

 domus, from a fine series of specimens obtained from the Lowick Limestones by the 

 Rev. L. Jenkinson. Having some doubt as to the correctness of this generic reference, 

 I asked Mr. Edgar Smith, of the Conchological Dej)artment of the British Museum 

 (Natural History Branch), to look at the specimens. He kindly wrote me as 

 follows : " At first sight I regarded the enclosed as true Lithodomi, but a com- 

 parison with that genus hardly justifies that opinion. The form is different, and 

 Lithodomus does not exhibit regular radiating lines. The subgenus Botula has 

 similar prominent umbones, but the general shape of the shell is diiFerent. There 

 is a group of Modiola, including M. glaherrima, D. Kr. ; M, arhorescens, Chemnitz ; 

 M. elegans, Gray, with which your shells might, I think, be associated." I have 

 therefore referred this species to the genus Modiola, but I must confess that I felt 

 very doubtful as to this species belonging to the Family Mytilida? at all until I was 

 thus reassured by Mr. E. Smith. I know of no specimens of this shell except those 

 in the Woodwardian Museum, Cambridge ; and I have to thank the authorities for 

 the loan of a fine series for examination, and also for permission to re-figure the 

 type specimen. Fig. 1, PI. VIII. The shell is, however, mentioned in the Survey 

 lists as occurring in the Middle Limestone of Wensleydale. This species differs from 

 all other Carboniferous Modiolas yet known in shape, the possession of well-marked 

 umbones, with a distinct anterior lunule ; the distance of its anterior adductoi* 



1 ' Geol. Surv. Memoir,' Country round Mallerstang, p. 202. 



