Mr. J. Lycctt OH Iftfl t/mus Tancmlia. 



TANCREDIA DONAC1FORM18. 



Sp. char. Shell subtrigonal, transverse, rather depressed, pointed 

 at the extremities ; umbones mesial or antero-mesial, small, 

 depressed ; anterior side attenuated, its superior margin rather 

 concave; posterior side truncated and gaping, its margin 

 straight, posterior to the ligament, and sloping obliquely down- 

 wards; an angle extends obliquely from the umbo to the 

 infero-posterior extremity; ligament short, external, hori- 

 zontal; margins of the valves not close-fitting and rather 

 irregular. 



Upon the principle that our choice of the typical example of 

 a genus should comprise the several peculiarities of the form in 

 a conspicuous manner, I prefer to select the present species to 

 illustrate Tancredia, and will adopt its hinge-characters in the 

 following amended description : — 



Hinge with an obtuse cardinal tooth in each valve, which is 

 received into a corresponding cavity in the opposite valve ; there 

 is also occasionally in the right valve a small anterior, and in the 

 left a small posterior, accessory tooth upon the elevated margin 

 of the cavity ; lateral teeth, one large posterior and approximate 

 in each valve, that of the left valve projecting, and received into 

 a depression formed by the tooth or callosity of the other valve. 

 Muscular impressions oval ; pallial impression simple, faintly 

 marked. There is no lunule : the margin of the right valve an- 

 terior to the umbo forms a thickened projecting fold which covers 

 the tooth of the other valve, and is received into a corresponding 

 receding portion of the margin of that valve, so that the junctions 

 of the valves anterior to the umbones have a sinuous flexure. The 

 lateral teeth are remarkably large in our typical species ; they are 

 never altogether absent, but are much depressed in some other 

 species ; and when this variation occurs in connexion with a de- 



