STANTON : THE MARINE CRETACEOUS INVERTEBRATES. 
15 
hinge line. Left valve convex ; anterior ear rounded, inconspicuous ; 
posterior ear broad, flattened and more or less mucronately produced at 
the extremity, though this is not preserved on most of the specimens ; 
surface of the whole valve, except the ears, with 25 to 30 fine radiating 
ribs, which tend to vary in size on the posterior portion. Right valve con- 
siderably less convex, with a deep byssal sinus under the anterior ear ; 
posterior ear broad and flattened ; surface marked by much finer and more 
numerous radiating lines than those on the left valve. 
An average specimen measures 1 1 mm. in height and about the same 
in greatest length, which is below the middle of the valve. The convex- 
ity of the two valves united is about 5 or 6 mm. 
The species is similar in form and sculpture to A. nebrascana Evans 
and Shumard, as figured by Meek, 1 from the Fort Pierre formation of the 
western United States. The subgenus Oxytoma ranges throughout the 
Mesozoic and is by some authors referred to Pseudomonotis instead of 
Avicula. 
Locality and position . — Abundant in the Ammonite (Belgrano) beds at 
mouth of canon of Rio Tarde four miles east of Lake Pueyrrydon. 
Gervillia hatcheri sp. nov. 
PI. Ill, Figs. 1 and 2. 
Shell very large, slender, obliquely produced, rather convex, with very 
thick test ; beaks terminal, pointed, not conspicuous ; ventral margin 
nearly straight from the beaks for about one-third the length of the shell, 
then broadly convex to the rounded posterior end ; dorsal margin slightly 
concave from the hinge to the posterior end of the shell ; posterior wing 
not preserved on the type but evidently narrow ; hinge area almost par- 
allel with the longer axis of the shell, broad with seven or eight large 
transverse pits or grooves for the ligament, and three or four obscure, ob- 
liquely elongated teeth ; surface marked only by lines of growth. 
Length 260 mm.; greatest breadth (at the beginning of the posterior 
third of the shell) 64 mm.; greatest convexity of single valve about 
28 mm., and thickness of test in anterior portion about the same. 
The only specimen in the collection is the left valve figured, which is in 
an excellent state of preservation, except that nearly all the posterior wing 
1 U. S. Geol. Surv. Terr., Vol. IX, p. 34, pi. 16, figs. 3a, 3b. 
