148 CARBONIFEROUS LAMELL1BRANCHIATA. 



binge-line is straight, a little shorter than the greatest length of the shell. The 

 umbones are obtuse, gibbose, elevated above the hinge-line, incurved, non- 

 contiguous, and situated in the anterior quarter of the shell. The umbones rise 

 very gradually from the surface of the shell. Behind, the umbonal slope is 

 continued downwards and backwards, till it becomes lost near the posterior inferior 

 angle. This slope is gently rounded, without any approach to a ridge. Above 

 and behind this the posterior slope is quickly compressed, so that it becomes 

 concave before it reaches the posterior part of the upper border. 



The surface of the valve is uniformly curved from above downwards, and from 

 before backwards ; an almost imperceptible flattening marking the byssal sinus. 



The interior of the shell is at present not known from British specimens, but 

 de Koninck states that the hinge is composed of three or four anterior teeth, 

 somewhat short and oblique ; posterior teeth very elongate, laminar, and sub- 

 parallel to the cardinal border. The area internal to the umbo is of moderate size. 



Exterior. — The surface of the valve is ornamented with fine concentric striae, 

 which are interrupted towards the lower border by strong, unequal, subimbricating 

 folds. Fine, regular, radiating strise are seen all over the shell, most strongly 

 marked in the posterior two-thirds. 



Dimensions. — PL X, fig. 10, measures — 



Antero-posteriorly . . . .34 mm. 



Dorso-ventrally . . . .20 mm. 



Elevation of valve . . . .8 mm. 



Locality. — England : the Carboniferous Limestone of Trey Cliff, Castleton. 



Observations. — There is much ambiguity about this species, which has been 

 quoted from several British localities ; and this uncertainty no doubt results from 

 the very great difference in the shells described by Prof, de Koninck in his two 

 great works. I cannot but think, both from the figures and description, that the 

 shells figured in his later work are of a different species from those depicted in the 

 former. I will quote and compare portions of the two descriptions which appear 

 to me to be contradictory. 



Area Lacordairiana, 1842 (p. 119). Parallelodon Lacordaireanus, 1885 (p. 157). 



Posterior border. — " Forteraent tronque" et " Un peu obliquement tronque et plus angle- 



formant avec l'inferieur un angle aigu." leux vers le bord ventral." 



Ventral border. — " Les bords cardinal et infe- " Celui-ce est arque." 

 rieur sont a peu pros paralleles." 



Oblique posterior slope. — " Son corcelet est " Carene oblique et obtuse." 

 borde par une forte carene oblique." 



Dimensions.— 7 .'. 8 : 18. 16 V 20 : 40. 



The strongly-marked keel and acutely angulated posterior inferior angle are 

 characteristic of P. cancellatus, to which, I am of opinion, the shell described in 



