312 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



shell, without an umbilical callosity, and with a more produced spire. Some 

 of the fossil species, however, show a very slight tendency to develop a small 

 spiral ridge within the umbilicus; though this is not always observable, even 

 in different individuals of the same species in which it sometimes occurs. 



It would be very difficult to determine the exact geological period at 

 which the genus Lunatia was first introduced, even if we could be sure that 

 we know all of the fossil species, because the shells of several extinct groups 

 closely resemble it in form and general appearance, while we know little or 

 nothing of the nature of the operculum of any of these ancient species. It may 

 be regarded as almost beyond doubt, however, that it did not exist during 

 palaeozoic times ; since even the Permian and Carboniferous forms most nearly 

 resembling it, and often referred to Natica by palaeontologists who make no 

 distinction between that group and Lunatia, belong to Naticopsis, short forms 

 of Macrocheilus, and perhaps other extinct genera. Some of the Triassic 

 so-called Naticas may possibly belong to this genus, and a few of the Jurassic 

 species closely resemble it; but it is, perhaps, not until we ascend to the 

 Cretaceous rocks that we meet with forms agreeing so closely that no distinc- 

 tion can be made, at least from the shell alone. It seems also to be well 

 represented through the Tertiary formations; and some forty or more species 

 are known in our existing seas. The latter are found mainly in cold or tem- 

 perate climates, though a few occur in more southern latitudes. Some of the 

 existing species attain to quite large sizes; L. heros (= Natica lieros, Say) being, 

 according to Dr. Gould, sometimes found as much as four and a quarter inches 

 in length, and three and a half in breadth. 



I see Dr. Stoliczka proposes, in his Palaeontology of lndia ; vol. II, page 

 296, to make the name Euspira of Agassiz, published in the German transla- 

 tion of Sowerby's Mineral Conchology in 1837, replace Dr. Gray's name 

 Lunatia, published in 1847, and now in general use for this genus ; and I am 

 not altogether sure that this may not be correct. Professor Agassiz did not 

 designate any particular species as the type of Euspira; but the first species 

 mentioned or described by him in connection with this name was the extinct 

 Natica glauconoides, Sowerby, which, so far as can be determined by the shell 

 alone, without the animal or operculum, would seem to be a Lunatia. Farther 

 on, in the same translation, however, he refers to Euspira species apparently 

 belonging to the older genera Straparollus, Montfort, 1810, and Ampullina, 

 Lamarck, 1813; while he also includes others apparently belonging to the 



