116 PALAEONTOLOGY OF THE UPPER MISSOURI. 



of the upper oblique slope of the whorls, together with its revolving lines, give it 

 much the appearance of some types of the Melaniidce ; while its small but unmis- 

 takable umbilical pit, and continuous peritreme, together with the expression of 

 the aperture, indicate nearer affinities to the Viviparidm. It is perhaps most nearly 

 allied to the genus Liaplax of Troschel, but differs from the type of that genus 

 (Paludina sulcarinata, Say) in having a much less ventricose and proportionally 

 smaller body whorl, more attenuate spire, and numerous thread-like revolving lines, 

 instead of a single carina ; while the posterior extremity of its aperture is suban- 

 gular instead of rounded, in consequence of the oblique flattening of the upper part 

 of the body volution. 



Lioplacodes Yeternus. 



Melania (Potadoma) veterna, MEEK & HAYDEN, Proceed. Acad. Nat. Sci. Phila. Deo. 1861, p. 444. 

 Lioplacodes veternus, MEEK, Smithsonian Check List, Jurassic fossils, 1864, 29. 



Shell with spire elevated ; volutions about six, very convex, rounded at the middle, and often obliquely a little 

 flattened above ; suture deep ; aperture obliquely oval, subangular behind, and rather narrowly rounded and faintly 

 sinuous in front, much more prominent or convex on the outer than the inner side ; 

 inner lip disconnected from the coluinella, so as to leave a small umbilical chink ; 

 outer lip broadly sinuous in outline near the middle of the aperture or slightly above, 

 and most prominent below. Surface marked by strong flexuous striae of growth, which 

 are crossed by more or less distinct thread-like revolving lines, some four or five of 

 which, near the middle of the body whorl, are larger aud more widely separated than 

 those below. 

 Lioplacodes veternus. Length, 0.77 inch ; breadth, 0.50 inch ; apical angle convex, divergence about 47. 



This interesting species was at first placed by us, with considerable doubt, in the 

 genus Melania, as that group was then understood by most conchologists. The 

 necessity for restricting that name, however, to such forms as M. amarula, has 

 become more apparent from late investigations, while the various recent American 

 species resembling our shell have been distributed into several genera, In endea- 

 voring to determine to which of these our species is most nearly allied, by clearing 

 away the matrix with care from about the aperture, it has been found, quite unex- 

 pectedly, to present characters, as already stated, showing affinities to the Vivipa- 

 ridre, rather than to the Melaniidce. And yet it differs from the known genera of 

 that family to such an extent that it has been thought desirable to regard it as the 

 type of a new genus. 



Locality and position. Head of Wind River, Dakota Territory, from beds referred 

 provisionally to the Jurassic system. Associated with Neritella Nebrascensis. (No. 

 1978.) 



CLASS CEPHALOPODA. 

 ORDER Tetrabrauchiata. 



FAMILY AMMONITE) J3. 



Shell involute, spiral, variously curved, or straight; outer or last cham- 

 ber large. Aperture varying in form with the genera and species ; lip 

 often more or less produced on the outer or dorsal side, sometimes hooded, 



