INVERTEBRATE PALAEONTOLOGY 29:> 



A nisoin you sexsulcatus, m. & H. 



Plate 18, 6gs. 8, a, 6. 



lhliiiKi texsulcaUu, Meek ami Hayden < l>.">(i). Proceed. Acad. Nat. s.i. 1'liilacl., VIII, 68. 

 Anisomyon sacsulcatus, Meek and Haydeii (I860), Am. Jour, s i ■ i . . XXV11I cJ.l ser. I. :!.">. 



Shell oval or elliptic-patelliforni, depressed, apparently a little broader 

 posteriorly than in front; apex located between the middle and the anterior 

 end; lateral slopes nearly straight, diverging from the summit at an angle <>t 

 about 100°; anterior slope concave, and having (on internal casts) two rather 

 distinct shallow grooves extending from the apex to the anterior-lateral 

 margins; posterior slope a little convex, and showing on casts four shallow 

 grooves, which radiate from the apex to the posterior and posterior-lateral 

 borders; (external surface unknown). 



Length, about 2.65 inches; breadth, near 1.29 inches; height, 0.4H 

 inch. 



The apex of the only specimen of this shell we have seen is broken 

 away, but has the appearance of having been abruptly attenuated and recurved, 

 as in the other species. The six radiating grooves seen on the cast seem to 

 have been produced by ridges on the inside of the shell, corresponding 

 exactly in position with the impressed lines on the exterior of A. borealis. 

 Whether or not there were corresponding grooves on the exterior surface, 

 over those seen on the cast, we have not been able to determine, not having 

 seen the outside of the shell. The muscular impression, as seen on the cast, 

 seems to present the same characters as that of the other species. 



This shell differs from all the analogous forms known in these rocks, in 

 the possession of the radiating grooves on the internal cast, mentioned in the 

 description. It is also much more depressed than any of the other species 

 attaining so large a size. 



Locality and position. — Yellowstone River, 160 mile; from its nth; 



in beds containing a blending of the fossils of the Fort Pierre and Fox Hills 

 groups of the Upper Missouri Cretaceous. 



