90 CONCHOLOGY. 



perforated ; the superior valve like a Patella, with a summit 

 more or less inclined towards the posterior side. 



O. Norwegica. The Norwegian Orbicula. PI. 17, fig. 4. 



Upper valve in the form of a depressed cone, with a sum- 

 mit produced and pointed. 



2, Terebratula. Twelve species. 

 Taken from the Anomia, and with great propriety, as its 

 characteristic differences are very great ; the perforation of 

 the Anomia is always in the smaller valve, which is attached 

 to the larger by a cardinal ligament, while in the Terebrat- 

 ula the perforation is always in the larger valve, which is 

 connected to the smaller by teeth at the hinge. In some 

 the valves are smooth, and in others grooved longitudinally. 

 Shell thin, equilateral, sub-triangular, inequivalve ; one of 

 the valves larger and more convex than the other, prolonged 

 behind by a sort of heel, sometimes recurved, and pierced 

 by a round hole at its extremity ; frequently sloped more or 

 less by a cleft of variable form ; the opposite valve smaller, 

 more flat, sometimes formed like a lid, having in the inte- 

 rior a system of support variable in form and complication 

 in every true species ; hinge limited, prominent, and formed 

 by two articulating surfaces of one valve placed between 

 corresponding projections of the other ; a kind of tendinous 

 ligament issuing from the sloping cleft of the shell, by which 

 it is attached to marine bodies. 



Terebratula vitrea. Terebratula pisum. 



T. dilatata. T. globosa. 



T. rotunda. T. sanguinea. 



T. flavescens. T. caput-serpentis. 



T. dentata. T. truncata. 



T. dorsata. T. psittacea. 



T. dorsalis. The dorsal Terebratula. PI. 17, fig. 1. 



The summit of the large valve pierced with a round hole, 

 very circumscribed ; grooved longitudinally ; valves as if 

 cut sloping in the middle line. 



