56 SYRIAX MOLLUSCAX FOSSILS. 



■which runs almost vertically downward from a point a little heJiind the beaks. 

 Shall the diagnosis of Cucullella be clianged to include the present species, 

 or shall the species be excluded from the group ? 



Zittel (Die Bivalven der Gosaugebilde in den Nordostlichcn Alpen, 

 Wiener Akademie, Denkschriften Math. Naturwiss., XXIY, pp. 102-164, 

 1805), Meek (Invertebrate Cretaceous and Tertiary Fossils of the Upper 

 Missouri Country, p. 98, 1876), and some others, following Deshayes, have 

 considered the shorter and genei'ally truncated side of the shell of Nucula 

 to be the posterior rather than the anterior part, the beaks to be directed 

 backward, and what seems to be the lunule as really corresponding- with 

 the escutcheon (area) of other types. But Zittel (Handbuch, I. Band, 

 2. Abtheilung, p. 52, 1881) has returned to the older and more common 

 view, which we have followed in our description of this species. 



N. c/jlindrica M'Coy (Synopsis of Carboniferous Limestone Fossils of Ire- 

 land, p. 69, PI. xi, fig. 26, 1844) is a closely related species. Finally, N. sub- 

 mucronala Conrad (Official Report, p. 213, PI. ii, fig. 14) based upon a cast 

 much larger than our specimens, may possil^ly be identical with this species. 

 But as his figure and description of the cast do not represent an internal 

 ridge to be present, and as specimens with the test were unknown to Con- 

 rad, we have regarded it as necessary to publish our examples under a 

 new name. Coll. Bird. 



Localil/j and Fosilion. — The cast above mentioned and specimens broken 

 to show the mode of mineralization have the same peculiar pyritiferous com- 

 position which was found in the Jurassic Ammonites from the Ornati Clay of 

 Mejdel esh Shems. Of the different collections considered in this paper, no 

 fossils known to be Cretaceous exhibit this kind of mineralization. We may 

 therefore infer that the species is probably from the locality and formation 

 which furnished the Ammonites referred to. 



Leda decussata, sp. nov. 



Plate VI, tigs. 4 a, b, c. 



Testa ovato-suUriquefra, antice inflata, posiice atlcnuaia, rodraia ; xmihones parvi, 

 incurri, contigni, medium pmne tcstam tenentcs ; lumda acute elliptica, fere plana, 

 ohscure marginala ; area profunde cxcavata, lira edila Ma rotiindala tcnniiuda : an- 

 ffiilits cardmalis obtumis : superficies striis concentricis regularibus crcbcrrimis, aliis 

 radiantibus minidis decussatis, inscidpia, sub vilro vemista. 



