110 CARBONIFEROUS LAMELLIBRANCHIATA. 



of the umbo obliquely downwards to join the anterior edge of the shell. At the 

 junction of the inferior border of the hinge-plate and the anterior edge is a shallow 

 smooth cavity, bounded below by a more or less well-developed ridge or plate, 

 which marks it off from the rest of the interior. This cavity is the hollow for the 

 insertion of the anterior adductor muscle. 



Just below the hinge-plate, as far as can be judged from fragments, about the 

 centre are two or three pit-like cavities, probably the site of the attachment of 

 the byssal and pedal muscles. 



The posterior adductor muscular-scar is large, obovate, and situated in the 

 upper and posterior quarter of the shell. The mantle-line is entire and remote 

 from the margin of the shell. The byssal notch is high up on the anterior surface. 



Exterior. — The surface of the shell is ornamented with fine striae and lines of 

 growth, which become markedly imbricate as they pass over the acute ridge. All 

 of these start close to the umbo, andj following the contour of the shell, terminate 

 in the upper border. On the anterior surface, near the edge of the valve, the 

 striae become very well marked, and may be traced over the edge of the hinge- 

 plate to be continuous with the cartilage-striae on it. 



Dimensions. — Fig. 2, PL III. The only complete specimen known from the 

 Redesdale Ironstone, though much crushed, measures — 



Length of hinge-line (restored) . . .48 mm. 



Greatest lateral width . . . .50 mm. 



From umbo to posterior inferior angle . . 82 mm. 



Locality. — The Redesdale Ironstone, Redesdale. ? Alnwick Moor. A frag- 

 ment of a large shell, probably of this species, has been obtained from the 

 Yoredale Rocks of Garsdale, by Miss Jane Donald. The Carboniferous Limestone 

 of Derbyshire ; and Narrowdale, Staffordshire. 



Observations. — The knowledge of the characters of this fine new species is due 

 to the indefatigable researches of Mr. John Dunn of Redesdale, who, by diligently 

 collecting numerous fragments, has enabled the important points in the internal 

 anatomy to be recorded. Only one specimen in at all a fair state of preservation 

 has been found ; this was presented by Mr. Dunn to Mr. Howse, of Newcastle-on- 

 Tyne, who kindly permitted me to have drawings made. 



The various fragments figured on PI. Ill show the essential particulars of the 

 anatomy of the species ; but, owing to the great depth and angularity of the 

 specimens, it has been somewhat difficult for the artist to indicate the correct 

 measurements. For example, the distance between the angle and the anterior 

 edge of the shell of fig. 1 is fully as great as that shown in the specimen fig. 3, 

 where the anterior surface is viewed from the flat. 



The position of the insertion of the anterior adductor scar is well seen in 

 figs. 1 a, lb, 5, and 11. The overlapping terminal umbo is seen in PI. Ill, fig. 3. 



