PRODUCTUS. 231 



Sp. Char. Shell small, almost circular, about as wide as long ; ventral valve gibbous ; 

 beak vaulted, but not projecting over the hinge-line, which is about as wide as the greatest 

 breadth of the shell ; ears small ; dorsal valve concave, following the curves of the opposite 

 one ; valves externally marked with regular longitudinal, undulating thread-like striae, and 

 small more or less defined irregular concentric wrinkles. Delicate spines rise likewise here 

 and there from the surface of the valves, and are more numerous on the ears close to the 

 cardinal edge. 



Length 6, width 6 lines. 



06s. Having sent British specimens of this shell to Prof, de Koninck, they were 

 declared to be identical with his Prod, undiferus. This small species does not appear to 

 attain the proportions of P. undatus, which it most resembles ; its ribs appear to be 

 proportionately smaller, and the wrinkles, where these exist, are never so large or regular 

 as in P. undatus. Its margin appears also to have been broad and regularly curved in 

 perfectly preserved specimens, as represented in PL LIII, fig. 6. This little shell was 

 discovered by Mr. Burrow in the Lower Scar Limestone of Settle, the only British locality 

 at present known. On the continent, it was found by Prof, de Koninck in the Carboni- 

 ferous Limestone of Vise, as well as in the shales of Tournay, in Belgium. 



Productus Nystianus, De Koninck. Plate LIII, fig. 9. 



Pkoductus Nystianus, De Koninck. Descrip. des Animaux foss. du Ter. Carb. de Belgiquej 



p. 202, pi. vii 1)ls fig. 3 ; pi. ix, fig. 7, and pi. x, fig. 9. 1843. 

 Also Monographie du genre Productus, pi. vi, fig. 4, and pi. xiv, 

 fig. 5. 



Sp. Char. Shell rather small; hinge-line straight, and as wide as the greatest width of 

 the shell ; ventral valve geniculated, semicircular, and much flattened on the posterior or 

 visceral portion, abruptly bent towards the margin ; beak very small, and hardly produced. 

 The visceral portion is marked by numerous more or less regular undulating concentric 

 wrinkles, interrupted here and there by projecting tubercules, while the anterior or bent 

 portion of the valve is ornamented with small longitudinal ribs. A row of curved spines 

 rise from and project over the cardinal edge. Dorsal valve almost flat on the visceral 

 portion, bent near the margin, and ornamented as in the opposite one. Interior 

 unknown. 



Length 6, breadth 8 lines. 

 Obs. Three or four examples of this interesting species, completely agreeing with those 

 represented by Prof, de Koninck in Plate XIV, fig. 5, of his 'Monographie/ were discovered 

 for the first time in England by Mr. Burrow, but none of them assumed the tubuliform 

 prolongations represented by my Belgian friend in PI. VI, fig. 4, of the above-named work. 

 Prof, de Koninck informs us that when adult and fully developed the shell assumes 

 an entirely different aspect, viz., that a portion of the prolongation of the larger or ventral 



