100 rAL^.ONTOLOG^'. 



central region is concave on the inside, and the posterior convex and pierced 

 by the fissure, which is rather short, witli its inner end about equidistant 

 from the posterior margin and from the middle of the valve; though its inner 

 end terminates at the center of growth, as may be seen by the concentric 

 arrangement of the fine obscure strise of growth observable on the interior. 



Antero-posterior diameter, 0.62 inch; breadth, 0.58 inch. 



The species of this genus are so difficult to disting-uish without a good 

 series of specimens, that I have been unable to satisfy myself that this is 

 new, or to identify it with any known species, and therefore merely give a 

 figure of it, as one of the forms found in the rocks at the locality men- 

 tioned below. If new, however, it may be called D. Utahensis. 



Locality and position. — Weber Canon, Wasatch Range; Triassic. 



lAMEI.LIBRANCHIATA. 



PTERIID^. 



Genus IIALOBIA, 13ronn. 



Halobia (Daonella) Lommeli, Wissmauii. 



Plivto 10, fig 5. 



Halohla Lommeli, Wissmaiin (1841), Beitr. Pefctef., IV. Heft, 22, tab. IG, fig. 11.— 

 Uorness (1855), Densk. Kais. Akad. Wis.seiiscJi., IX, 52, taf. ii, fig. 17.— 

 Zittel, Fossile Moll, uiid Ec^biuoderinen aus NeuSeelaiul, 27, taf. vi, figs. 

 1 a, h, c— Stoliczka (ISfiG), Mem. Geol. Survey India, V, 44. 



Avicula pectiniformis, CatuWo (1847), Prodr. Pal. Alpi Veu., 73, pi. i, figs. 1, 2, 3. 



Posidonomya Lommeli, d'Orbiguy (1840), Prodr. de Paleont. Stratigr. Univ., I, 201. 



f Ealohia ? duliia, Gabb (1SG4), Geol. Survey Galifornia, I (Palreout.), 30, pi. 5, figs. 

 28 a, h. 



Daonella duhia, Mojsisovics (1874), Ueber die Trascb. Pelicyp.-Gatt. Daouclla imd 

 Ealobia, 22. 



Shell truncato-subcircular, very much compressed, and very slightly 

 obHque, nearly or quite equivalve, and but slightly inequilateral, the ante- 

 rior side being a little shorter than the other; hinge somewhat shorter than 

 the valves; anterior and posterior margins intersecting the hinge at obtuse 

 angles, the latter more obliquely than the other, both rounding into the 

 more or less regularly rounded base; posterior basal margin more promi- 

 nent tlian the anterior; beaks very small and inconspicuous, with their 



