146 CARBONIFEROUS LAMELLIBRANCHIATA. 



Dimensions : 



The type specimen, 

 PI. X, fig. 5. PL X, fig. 7. 



Antero-posteriorly (estimated) . . 38 mm. ... 29 mm. 



Dorso-ventrally . . .21 mm. ... 18 mm. 



Elevation of valve . . . 12 mm. ... 5 mm. 



Localities. — England : the Carboniferous Limestone of Settle, Yorkshire, and 

 Park Hill, Derbyshire. Ireland : Blacklion Upper Limestone. The species was 

 described by M'Coy without a locality. The type of this species came from 

 Derbyshire, but no locality was mentioned by Martin. 



Observations. — This species is distinguished from all others by the strength of 

 the radiating lines, which become distinct ribs, and are marked even on the 

 interior of the valves (as seen in casts), and by the diagonal ridge between the 

 body of the shell and the dorsal slope, which is stronger in this species than in any 

 others. M'Coy calls attention to this fact in his description of Byssoarca costellata 

 (p. 72), which is as follows : — " Width twice and a half the length, very convex; 

 beaks rather large, prominent ; anterior end short, pointed ; posterior end 

 rectangular ; posterior slope separated from the body of the shell by a very steep 

 ridge ; entire surface ornamented with equal, rounded, radiating ribs." The type 

 specimen seems not to be in the Griffith Collection at the Dublin Natural History 

 Museum ; and, although it seems to have been immature, there is no difficulty in 

 recognising such a well-marked shell as belonging to Martin's species. 



The shell upon which Martin founded his species, " Area cancellata" and which 

 was subsequently figured by Sowerby (op. suj). cit.) is probably one preserved in 

 the Sowerby Collection at the British Museum (Nat. Hist.) ; and by the kind 

 permission of the authorities I am able to figure this specimen, PI. X, fig. 7. It 

 is somewhat more fully grown than M'Coy's specimen, and has the test well 

 preserved. The acute ridge and truncated posterior end are well marked. 

 Sowerby says of it, " From the Derbyshire Limestone, apparently the same shell 

 that was figured by Mr. Martin." Martin says of it, " A fossil shell, the original an 

 Area. The hinge straight, extending the whole breadth of the shell. Valves 

 equal, somewhat rhomboidal ; their anterior extremity sloped and angular, the 

 other end more rounded ; surface cancellated with longitudinal and transverse 

 equal stride. The beaks small, placed nearer the rounded extremity." The shell 

 is not to be identified from the figure ; but this is immaterial, as the original of 

 Sowerby's shell is still in existence. The description given by that author is, 

 " Twice as wide as long, convex, longitudinally and transversely sulcated ; 

 anterior [posterior] side defined by a keel, truncated posterior side rounded ; 

 marginal sinus short, but deep ; beaks nearly close." 



M'Coy recognised this species as occurring in the Irish Carboniferous 

 Limestone, and even goes so far as to give a diagnosis (p. 71), but without 



