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 appenpicuLata, J. Sowerby. Tab. XIV, fig. 3, a—e. 
Arca apprnpicuLaTa. J. Sow., Min. Conch., t. 276, fig. 3, 1820. 
— pupuiicata. J. Sow. Min. Conch., t. 474, fig. 1, 1824. 
— = Morris. Catal. Brit. Foss., p. 185, 1854. 
— uacTEA. Solander, in Brand. Foss. Hant., t. 8, fig. 106, 1766. 
— suuictcosta. Nyst. Coq. foss. Belg. p. 257, pl. 18, fig. 9, a, 6, 1843. 
Spec. Char. A. testd elongatd, subtrapezoidali, gibbosuld, inequilaterali, equivalvi, 
radiatim striata vel costulatd, concentricé decussatd ; costulis duplicatis vel bifurcatis : 
siphoni-regione longiore, oblique truncata ; pedi-regione superné angulatd, inferné rotun- 
data ; umbonibus subprominentibus, tncurvis ; ared connexiis 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, ths of an inch. 
Localities. Barton. 
Belgium, Viiermael, 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 strie 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. 
