OCR Text |
Show 410 APPENDIX B.- PALEONTOLOGY. Fig. 4 b, the dorsal valve of a larger individual, having six plications on each side of the mesial sinus. The surface is marked by distinct, undulating, concentric striae, which are again crossed by finer thread- like elevations, and which appear to have been the bases of short spines. Loc. Missouri River, near Weston. ' 9 n ~ SPIRIFER TRIPLICATA, Hall, ( n. sp.) t?/ ^ l. : ^ \ . . .. PLAnILFxo. 6abo. *'''^ 7- Shell subquadrangular; dorsal valve more gibbous than the ventral; area nearly linear; beak of dorsal valve small, acute, and closely incurved; mesial depression shallow in the upper part, but becoming deeper and expanded toward the base, and produced in front; entire surface, including the mesial sinus and elevation, covered with fine plications, which, being simple in their origin, soon divide into three, which are continued to the base without further division. This species differs from the S. strtatusj Sowerby, in its form, and in the circumstance that the plications are less subdivided toward the margin of the shell. Fig. 5 a, ventral valve, and area of the dorsal valve. * Fig. 5 b, dorsal valve of the same individual. Fig. 5 c, profile view of the same. The form is somewhat distorted by pressure. Loc. Missouri River, above Weston. CHONBTES VARIOLATA, ( D'Orb. sp.) De Koninck. C. PLATBIII. FIO. lab. This species bears the essential characteristics of those figured and described by De Koninck, though it is larger than most of his figures. The broad, scarcely defined mesial depression of the dorsal valve gives a straight or slightly sinuous outline in front. This fossil is associated with several of the preceding species, near Weston on the Missouri River. |