OKBTAOEOUS FOSSILS. 433 



PULMONIFERA. ? 



SIPHONARIID^E ? 



Genus ANISOMYON, M. and H. 



A few examples only of this genus are found in the collections from 

 the Black Hills. These few represent four of the species previously 

 described from this region. The generic characters of the group are well 

 marked on all the species we have examined, but there appears to be some 

 doubt as to its family relations. Mr. Meek's latest expressed opinion seems 

 to be that they are most nearly allied to the Siphonariidce, and therefore that 

 they are pulmoniferous in their structure. To this conclusion the break in 

 the right side of the muscular imprint would naturally lead, for although 

 the interruption is not large, and often quite slight, still it is an interruption, 

 and would only occur through some organic feature of the animal ; the 

 natural inference would therefore be that it was for the protrusion of an 

 air-conducting siphon, or at least that it indicated a step in that direction. 

 Still, so far as our examinations extend, there has been no depression of 

 the interior surface of the shell, as is usually the case in most if not all the 

 species of Siphonaria. 



ANISOMYON ALVEOLUS. 



Plate 12, fig. 20. 



Relcion alveolus M. & H., Proc. Acad. Nat. Sci. Phil., vol. 8, p. 68. 

 Anisomyon alveolus M. & H., Araer. Jour. Sci. and Arts, vol. 28, 2d ser., p. 35. 

 Anisomyon alveolus (M. & H.) Meek, Pal. U. S. Geol. Surv. Terr., p. 292, PI. 18, fig. 4. 



Shell small, elongate-oval or narrowly elliptical in outline, the length 

 about once and a half the width, with rounded anterior and posterior 

 extremities; apex a little in advance of the middle, minute and apparently 

 recurved; elevation of the shell, in our specimen, about half or a little 

 more than half as great as the greatest width, the surface of the sides and 

 ends very gently convex. The surface, judging from the exfoliated speci- 

 men, has been slightly marked by concentric undulations ; anterior muscu- 

 lar imprint on the right side long and narrow and a little obliquely placed.' 

 28 B H 



