BIVALVIA. 79 



the more inequilateral species have a large, open, and elongated space for connexus, the 

 linear portion diverges from the umbo at an acute angle ; and thus a greater strength is 

 given to these lines than if they were spread over the entire surface. If the same mode of 

 divergence existed in the siphonal region of these shells, the linear portions of the ligament 

 would be extended at an angle so obtuse, that they would be almost useless for a ligature, 

 and therefore another set of divergent lines is formed on this part of the dorsal area, by 

 which a greater tenacity is obtained and the firmer union of the two valves is secured. 



1. Arca appendiculata, /. Sowerby. Tab. XIV, fig. 3, a — c. 



Arca appendiculata. J. Soic, Min. Conch., t. 2/6, fig. 3, 1820. 



— duplicata. J. Sow. Min. Conch., t. 474, fig. 1, 1824. 



— — Morris. Catal. Brit. Foss., p. 185, 1854. 



— lactea. Solander, in Brand. Foss. Hant., t. 8, fig. 106, 1/66. 



— sulcicosta. Nyst. Coq. foss. Belg. p. 257, pi. 18, fig. 9, a, b, 1813. 



Spec. Char. A. testa elongatd, subtrapezoidali, gibbosuld, inaquilalerali, eequivalvi, 

 radiatim striata vel costulatd, concentrice decussatd ; costulis duplicatis vel bifurcatis ; 

 siphoni-regione longiore, oblique truncatd ; pedi-regione superne angulatd, in/erne rotun- 

 datd ; umbonibus subprominentibus, incurvis ; area connexus magna, partim ornatd. 



Shell elongate, irregularly trapezoidal, tumid, inequilateral, equivalve, radiated with 

 prominent lines or riblets, decussated by lines of growth ; pedilateral margin angular 

 above and rounded below ; siphonal region angulated ; beaks rather prominent, with a 

 large area for the connector, partly lineated. 



Length, \\ inch ; height, |tbs of an inch. 



Localities. Barton. 



Belgium, Vliermael, Hoesselt, and Lethen {Nyst.). 



This species is in England restricted to the above locality, where it is not very 

 rare; and as I believe the four names above mentioned belong to one and the same species, 

 I have adopted appendiculata as being the older of the four. The peculiar appearance 

 which suggested the name appears to be a plain and naked space on the pedal side of the 

 area for connexus, which is free from any diverging impressed lines, while they are distinct 

 on the siphonal side ; at least I presume this to be so ; I am quite unable to see anything 

 like appendages to this part of the shell. The exterior of the valve' is ornamented with 

 radiating striae or riblets, and these generally duplicate, sometimes triplicate, on the outer 

 or older portion of the shell ; the lines of growth are prominent, decussating the interspaces 

 as well as the rays, which, in consequence, become somewhat nodulous, particularly over 

 the pedal region ; the dental area is furnished with teeth along the entire length ; they 

 are at nearly right angles to the hinge-line, but incline as they recede from the centre. 



