CRAGiN.] DESCRIPTIONS OF SPECIES. 55 
land sharp keel by union with the corresponding border region of the 
opposite valve; anterior umbonal slope only feebly angulated or 
shouldered; beaks narrow, high-arched, approximated, placed slightly 
in advance of the middle third; Hank region marked with concentric 
growth lines; posterior slope finely and closely striate. 
Measurements.— Height, 39 mm. ; length, about 48 mm. ; breadth, 
about ?A mm. 
Occurrence. — Two examples were collected by Doctor Stanton in 
foothills at the northwest end of Malone Mountain, about 2 miles 
bast-southeast of Finlay station. Neither shows the ornamentation 
of the anterior slope. 
The species is named for the late Prof. Antonio del Castillo, 
founder-director of the Geological Institute of Mexico. 
NUOULID^. 
Genus LEDA Schumacher. 
LEDA ? NAVICULA Sp. 11. 
Pi. VI, fig. 13. 
Shell fairly well inflated, elongate-ovate, anteriorly not shortened, 
posteriorly produced and gradually narrowed or subrostrate ; the 
anterior subhorizontal and posterior gently declined parts of the dor- 
sal side each forming a small concavity adjoining the beak, anterior 
bide rounded, base long and straight-convex for the greater part, its 
posterior part ascending obliquely; beak small and little salient, 
placed a little back of the anterior third. 
Measurements. — Height, 11.5 mm,; length, 25 mm.; breadth, about 
p.0 mm. 
Occurrence. — A cast of a left valve was found embedded in the 
rock fragment bearing one of the specimens of Unicardium semiro- 
tundum collected by Doctor Stanton at the anticline on the east slope 
bf Malone Mountain about 1 mile north of its southern end. 
A cast of a small, posteriorly produced lamellibranch, which was 
supposed in the field to be one of the Nuculidse and which may have 
been a Leda, was observed in Theta a mile and a half east of Malone 
station. It was accidentally destroyed in attempting to remove it 
from the matrix. A rough sketch of it, preserved in my notes, indi- 
cates a shell similar in form to that above described, but smaller. It 
pay be referred to Leda? navicula, but is represented as having the 
margins crenulated, while this character, if it belonged to the latter 
species, can not be distinguished in the type as preserved. 
