108 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



other in form and general appearance, that it is not always easy to distinguish 

 fossil species of the two groups from each other. When the pallial line, or 

 its impression on internal casts, can be seen, however, its more deeply sinuous 

 character in Yoldia is a good distinguishing character ; and the latter genus 

 also usually has a larger cartilage-pit than Nuculana. Again, the valves of 

 Nuculana are generally more depressed, more attenuated posteriorly, and, in 

 many cases, have the posterior umbonal slopes more prominent or angular. 

 There are likewise said to be differences in the animal of these two groups ; 

 Yoldia not having the ventral lobes seen at the posterior margin of the mantle 

 in Nuculana and Neilo. In the form of the shell, the last-mentioned genus 

 resembles some species of the section Portlandia; hut the pearlaceons inner 

 layer and internal cartilage of the latter will at once distinguish the shells 

 of these groups. 



From the genus Nucula, to which some palaeontologists, who are not 

 over-particular in regard to generic distinctions, still persist in referring 

 fossil species of Yoldia, the latter genus may be at once distinguished, not 

 only by the entirely different form and less nacreous character of the shell, 

 but by the non-sinuate pallial line of Nucula, to say nothing of the striking 

 differences of the animal in these groups, which no well-informed concholo- 

 gist now refers to the same family. 



The genus Yoldia seems to date back to Palaeozoic times. At least, we 

 have shells from the Carboniferous and Permian rocks that have exactly the 

 form and crenate hinge of the typical species. I am not quite sure that I 

 have seen the pallial sinus in any of these, though I think traces of it may 

 sometimes be seen on some of the casts of the interior. Some of the Triassic 

 and Jurassic species that have been referred by palaeontologists to Nuculana 

 (= Leda) probably belong here ; while several ot the species referred by 

 d'Orbigny and others, to Leda and Nucula, from the Cretaceous of France, 

 seem to be typical yoldias. 



This genus was also represented by several species during the Tertiary 

 epoch, and probably attains its greatest development in the existing seas. 

 The recent species are chiefly found in northern and antarctic seas, and occur 

 on the coasts of Greenland, Kamtschatka, Massachusetts, &c. 



