72 MOLLUSCA FROM THE GREAT OOLITE. 



Genus UNICARDIUM. IfOrbigny, 1847. 



Shell thin, convex, ovately oblong ; umbones contiguous, depressed ; hinge margin 

 elongated, nearly horizontal ; margins of the valves rounded, not close fitting, but without 

 any regular aperture. Hinge ligamentary, the ligament being external, supported by a 

 thin shelly lamina, which is partly internal, and extends posteriorly the length of the hinge 

 margin ; beneath the umbo is a small depressed tooth in each valve, but these are nearly 

 obsolete, and in the greater number of specimens cannot be distinguished. Muscular 

 impressions elliptical; pallial impression simple. The external surface is destitute of 

 ornament, but has large, concentric, irregular plications ; the substance of the test is very 

 thin. 



The three species which we give as examples of Unicardium, belong to an extensive 

 series of shells, several of which M. Agassiz has figured and described as Mactromyae, but 

 which are in fact perfectly distinct from another portion of the same genus, for which the 

 name Mactromya may perhaps be retained ; these latter are Mactromya mactroides 

 striolata, tenuis, and brevis ; these shells are distinguished by well-marked features, 

 externally they have an oblique posterior angle, internally they have an anterior, oblique, 

 elongated rib, and a large sinus in the palleal impression. 



Three other species of the same author, viz., M. globosa, aqualis, and rugosa, have a 

 figure much more convex, without any posterior angle ; internally they are destitute of 

 the anterior rib, and their palleal impression is simple ; the latter group should therefore 

 be removed from the Myada. M. D'Orbigny, (' Prodrome de Paleontologie,') has referred 

 this group, together with other shells, to his new proposed genus Unicardium, the type 

 of which is Corbula cardioides, of Phillips. Unicardium is described as resembling 

 Cardium, but having only a single cardinal tooth in each valve. 



Unicardium comprises a numerous group of species, several of which are so nearly allied 

 in form as to be with difficulty distinguished. They occur throughout the Lias, the lower, 

 the middle, and the upper Oolitic rocks of Europe, and it is probable that many species 

 remain undescribed ; they occur indifferently in beds of clay, in lias limestone, and in 

 shelly oolite, in the latter case the valves are always disunited, but in the lias and other 

 clays, and argillaceous limestones, the valves are invariably in apposition. Their habits 

 were not gregareous, but, on the contrary, they always occur sparingly, and from the 

 thinness of the test, have often sustained fracture or compression. From the borders of 

 the valves not being close fitting, and perhaps from a considerable amount of lateral motion 

 which the kind of union in the valves would permit, one valve frequently overwraps 

 the other, producing a mistaken appearance of inequality in the valves, which may have 

 led to the species first figured in the ' Geology of Yorkshire/ having been assigned to 

 Corbula. 



We regard Unicardium as presenting a considerable resemblance to certain species of 

 Lucina, and would arrange it in the Malacological series near to that genus. 



