STANTON : THE MARINE CRETACEOUS INVERTEBRATES. 
27 
Martesia argentinensis sp. nov. 
PI. VI, Figs. 3 and 4. 
Shell small to medium in size, elongate ovate or cuneate, very inflated, 
with a spherical aspect from the front, regularly tapering to the posterior 
end, which gapes rather widely, anterior hiatus shield-shaped, closed by a 
callum ; a single broad accessory umbonal valve present (form not accu- 
rately determined), other accessory valves unknown ; surface marked by 
fine regular lines of growth, parallel with the margins of the shell, and by 
two faint umbonal furrows that diverge slightly in passing obliquely from 
the beaks to the ventral margin just behind its most prominent portion, 
and are more prominent on internal casts than on the shell itself; the 
burrows in fossil wood, in which the shells are found, not lined with a 
calcarous shell. 
An average specimen measures 13 mm. in length; 7.5 mm. in height, 
and 8 mm. in greatest convexity. 
The two umbonal furrows in this species suggest its reference to Para- 
pholas, which is represented in the Cretaceous by somewhat similar forms, 
but that genus has paired umbonal valves, or a single one formed of two 
fused pieces. 
Locality and position . — From the Ammonite (Belgrano) beds at mouth 
of canon four miles east of Lake Pueyrrydon, represented by a dozen 
specimens, most of which are immature. 
Turnus dubius sp. nov. 
PI. VI, Figs. 5-8. 
Shell small, subglobose, or broadly ovate, gaping widely behind, and 
in front with abroad, shield-shaped hiatus, which in some specimens seems 
to be filled by a callum, its upper corners almost rectangular; beaks 
prominent, approximate, in front of the middle of the shell ; dorsal margin 
nearly straight, posterior end and venter very broadly rounded ; umbonal 
groove slightly oblique, narrow, inconspicuous, somewhat more prominent 
on internal casts than on the exterior, extending from the beak to the 
most prominent part of the ventral margin ; surface also marked by very 
fine, closely arranged, regular lines of growth that cross the umbonal fur- 
row obliquely and are sharply bent upward in front of it, parallel with the 
margin of the anterior gape. 
