50 PALAEONTOLOGY OF THE UPPER MISSOURI. 



are decidedly of the opinion, however, that it is more nearly allied to the latter. 

 It is true these shells have no internal cartilage pit, but we have ascertained that 

 they have the shell structure of the Pectinidoe, and not that of the Ptenidm. 



The annexed cut shows the structure of Avicidopecte^i amj^lus, Meek & Worthen, 

 as seen in a fragment placed in Canada balsam, under a magnifier of 350 diameters/ 



It will be seen that there is here no traces of the prismatic cellular structure of 

 the Ptei'iida;. The shell, on the contrary, is composed of very thin laminae, with 

 striated or corrugated surfaces as in the Pectinidce. These striaj are not parallel on 

 the different lamina?, nor on the opposite sides of the same layer, but arranged so 

 that on looking through several of these plates they are seen crossing each other 

 at various angles. From this structure, therefore, taken in connection with the 

 form and general appearance of these shells, it is manifest they belong to the Pec- 

 tinida;, or possibly to an intermediate group between that and the Pteriid(e. 



This genus was probably introduced during the deposition of the Devonian rocks. 

 It attained its maximum development during the Carboniferous epoch, and is also 

 represented in the Permian rocks, the deposition of which it seems not to have 

 survived. 



ATiculopecten ? 



(Plate II, Fig. 10.) 

 Comp. Pecten cleavelandicus, Swallow, Trans. Acad. St. Louis, I, Feb. 22, 1858, p. 182. 



We are in doubt whether or not this shell is identical with Prof Swallow's 

 species cited above, and consequently prefer not to describe it as new, though we 

 suspect it may prove to be distinct. It certainly does not appear to have presented 

 the same proportions, judging from his measurements, which make the height and 

 breadth of his species as 1.63 to 0.95; though we think there must be a typo- 

 graphical error in these figures, since P. cleavelandicus is described as being 

 " orbicular." It would also seem to differ from our shell in being " oblique." 



Locality and 2)osition. — Kansas ; near Chapman's Creek, eighteen miles above 

 Fort Riley. Permian beds. 



Aviculopecten IttcCoyi. 



(Plate II, Fig. 9.) 



Shell under medium size, broad suborate exclusive of the ears; not oblique, rounded on the ventral margin, and 

 liaving a moderately deep, rounded sinus under each ear. Hinge margiu sloping slightly from the beaks, and 

 equalling about three-fourths the breadth of the widest part of the valves below. Left valve gibbous ; umbo convex, 

 its sides couverging at an angle of about 850 to the apex ; ears nearly equal, the posterior one being a little larger 

 and more angular than the other. Anterior ear somewhat rounded at the extremity, separated from the swell of 



I The lines are too straight and regular in this cut. Wood engraving is not well adapted to the illustration of 

 such objects. 



