ARCACEA. 



717 



Nuada, in an internal pit. The shell is generally oval or elon- 

 gated, smooth, or marked with fine concentric lines ; there is no 

 ligamental area (as there is in Pectunadus), but the hinge carries 

 numerous small transverse teeth. The species of Tellinomya range 

 from the Ordovician to the Carboniferous, and the forms placed by 

 Hall under the genus Palceonedo are probably congeneric with the 

 above. 



The genus Nuadana (Ledci) is the type of another group of the 

 present family, which is distinguished by the fact that the shell is 

 more or less produced posteriorly, and also usually by a more or 

 less marked sinuation of the pallial line (589, a) ; for which reasons 

 these forms are sometimes considered as constituting a separate 

 family (Xuculanidtz). In Nuadana (Zeda) the shell (fig. 589, a) 



Fig. 589. — Types of NitcididcE. a, Interior of the right valve of Kuculana (Leda) lanceolata — 

 Pliocene; b, Cucullella ovata — Silurian; c, Yoldla striatida — Cretaceous, enlarged; d, In- 

 terior of the left valve of Yoldia myalis — Pliocene. 



resembles that of Nuada, but is rounded in front and produced 

 behind, while the pallial line is generally more or less indented. The 

 hinge has numerous small teeth on either side of a small central 

 cartilage-pit. Various species of the genus have been described 

 from the Palaeozoic rocks, beginning in the Devonian, and it is 

 abundantly represented in the Secondary, Tertiary, and Post-Ter- 

 tiary deposits, as well as by living forms. Yoldia (fig. 589, c and 

 d) resembles the preceding in most respects, but the teeth are comb- 

 like, and there is a large pallial sinus. The genus is perhaps rep- 

 resented as early as the Devonian or Carboniferous, but can hardly 

 be distinguished from Nuadana in the fossil condition. 



In the neighbourhood of Nuadana may be placed the Ordovician 

 genus Lyrodesma, in which there is an equivalve oblique shell, truncated 

 posteriorly, with an external ligament, and having a hinge of several 



