CONOCARDIUM IRREGULARE. 457 



small, triangular, separated from the body of the valve by a slight oblique 

 compression, pointed and gaping at the extremity. The inferior margin 

 descends downwards and backwards to the lowest point of the valve, and then 

 becomes rapidly bent, and passes upwards and backwards to the base of the 

 rostrum. The hinge-line is straight and prolonged, continuous with the upper 

 margin of the rostrum. The umbones are small, angular, pointed, and slightly 

 raised above the hinge-line; the rostrum long-pointed, springing from a very 

 elongate base, and supported by a triangular ribbed process of shell. The valve 

 has really no posterior surface, but a triangular lateral surface, the anterior and 

 posterior parts being inclined to each other at a very large angle, and meeting in 

 a strong vertical ridge, which passes from the umbo to the lower margin, the 

 posterior portion representing the posterior surface in other species of the 

 genus. 



Interior. — The details of the hinge and muscle-scars are unknown. 



Exterior. — The anterior half of the shell is smooth and marked by concentric 

 lines of growth, which are parallel with the lower border. The median part of the 

 valve is marked by a strong vertical, angular rib, which passes from the umbo to 

 the lower border, behind which are others, distinct and separate, but less strongly 

 marked, and diminishing in size from before backwards. The base of the 

 rostrum is also covered with short oblique ribs. Shell comparatively thick. 



Dimensions. — PI. LI, fig. 12, a specimen from Settle, in the Woodwardian 

 Museum, measures — 



Antero-posteriorly . . . .10 mm.-^ 



Dorso-ventrally . . . . 5 mm. 



From side to side (estimated from one valve) . 5 mm. 



Locality. — The Carboniferous Limestone of Settle, Yorkshire. 



Observations. — This very distinct and peculiarly marked species has been 

 described by de Koninck, in both his great works, from the Carboniferous 

 Limestone of Vise. I have only met with two specimens of the species in Grreat 

 Britain, and both are in the Burrows Collection of the Woodwardian Museum, 

 Cambridge. 



The absence of ribs on the anterior part and the few strong ribs on the 

 posterior part, separated by the first rib, which makes a strong ridge dividing the 

 shell, the absence of a posterior surface, and the elongate ribbed base of the 

 rostrum are important characters for the recognition of the species. G. rostratum 

 has somewhat analogous shape, but has not the peculiarly characteristic ribs of 

 G. irregulare. M. Eraipont, who, it seems, furnished the description for this shell 

 for the latter of de Koninck's great works, has not, I think, pointed out the absence 



1 N.B. — It is difficult to differentiate between the rostrum and the prolonged posterior end of 

 the valve. 



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