PARALLELODON VERNEUILIANUS. 133 



Specific Characters. — Shell only of medium size, transverse, subtrapezoidal, 

 slightly oblique, and only slightly swollen. The anterior end is small, compressed, 

 and narrowed in its dorso-ventral diameter. Its border is semicircularly rounded 

 below at the expense of the anterior inferior angle, but is straight above, and 

 makes a right angle with the hinge-line. The inferior border is slightly curved 

 with its convexity downwards, more so at either extremity. The posterior 

 border is straight, obliquely truncate from above downwards and backwards, 

 forming an obtuse angle above with the hinge-line, and a blunted acute angle 

 below. The hinge-line is straight, and as long as the greatest transverse diameter 

 of the shell. 



The umbones are small, swollen, pointed, directed forwards aud inwards, only 

 slightly raised above the hinge-line, and situated at the junction of the anterior 

 and middle thirds of the cardinal border. They are close but not contiguous, aud 

 are separated by a narrow ligamental area. Passing downwards and backwards 

 from the posterior border of the umbo is a well-marked angular ridge, which 

 becomes less conspicuous as it approaches the posterior inferior angle, and 

 separates the body of the shell from the compressed and hollowed dorsal slope ; 

 in front of the diagonal ridge the valves are slightly but regularly convex. 



Interior. — The details of the interior have not been observed in British 

 specimens. De Koninck (op. supra cit., p. 157) states that the hinge possesses 

 three elongate posterior teeth, subparallel with the cardinal border. 



Exterior. — The surface is ornamented with several (ten to fifteen) subequi- 

 distant, prominent, rough, imbricating, concentric lamellae ; separated by slightly 

 hollowed concentric spaces, wider behind than in front, which become less concave 

 and flatter towards the lower border. These spaces are covered by regular, close, 

 fine, radiating striae, interrupted by each concentric lamella, visible to the naked 

 eye, and becoming coarser on the posterior slope. In the neighbourhood of the 

 byssal sinus there is considerable irregularity to be seen in the concentric lamella?. 



Dimensions : 



Antero-posteriorly. Dorso-ventrally. Laterally. 

 PL IX, fig. 13 measures 25 mm. 15 mm. 11 mm. 



PI. IX, fig. 14 . .25 mm. 15 mm. — 



Localities. — England : The Carboniferous Limestone of Castleton, Derbyshire ; 

 Wetton, Staffordshire. Ireland : The Carboniferous Limestone of Little Island, 

 co. Cork. 



Observations. — This is a readily distinguished species, and is easily diagnosed 

 from all others by the very distinctive ornament. It forms, however, a natural 

 link between P. squamifer, which has, as a rule, only the strong concentric 

 lamellae common to both species, and the other species of the genus, as 

 P. reticulatus and P. tenuistriatus, which possess a radiating ornament largely in 



