Fauna of the Morrow Group 
169 
not distinct from visceral region; lateral margins meeting the 
hinge-line at right angles but curving gracefully into anterior 
margin. 
Surface of both valves when well preserved marked by very 
fine, discontinuous costae which are seen in worn specimens to 
be due to the linear arrangement of small, somewhat prostrate 
spines thickly distributed over the surface of the valve, whose 
presence is known in the material at hand solely from the 
punctate scars marking the points of attachment. Concentric 
ribs cross the umbonal region of both valves at irregular inter- 
vals while toward the anterior margin the concentric growth 
lines which mark the surface become quite conspicuous and 
somewhat undulatory. 
Remarks, It is evident that this form belongs to the group 
of shells typified by P. longispiria (Sowerby) but its distinctive 
characters are so striking as to warrant the erection of a new 
species to receive it. Most notable, probably, are the absence 
of definite auriculations in either valve and the fairly sharp 
boundary between the cardinal slopes and main flanks due to the 
posterior deflection of the former. 
Horizon and locality. Brentwood limestone : near Fayette- 
ville, Arkansas (Station 152). 
Pustula nebraskensis (Owen) 
Plate IX, figures 6, 7. 
1852. Productus nebrascensis. Owen, GeoL Rep. Wis., Iowa, and Minn., 
p. 594, tab. 5, fig. 3. 
Carboniferous limestone: Bellevue, Missouri River, Nebraska. 
1855. Productus Rogersi. Norwood and Pratten, Jour. Acad. Nat. Sci., 
Phil., 2nd ser., vol. 3, p. 9, pi. 1, figs. 3a-c. 
Coal Measures: Near Huntsville, Missouri. 
1855. Productus nebrascensis. Norwood and Pratten, Jour. Acad. Nat. 
Sci., Phil., 2nd ser., vol. 3, p. 21. 
Coal Measures: Crossing of Big Nemahaw, Nebraska Territory. 
1856. Productus rogersi. Hall, Pac. R. R. Rep., vol. 3, p. 104, pi. 2, 
figs. 14, 15. 
Carboniferous limestone: Pecos Village, New Mexico. 
1859. Productus Rogersi. Meek and Hayden, Proc. Acad. Nat. Sci., Phil., 
p. 26. 
Upper Coal Measures: Kansas River Valley, below mouth of Blue 
River, Kansas. 
