8 TESTACEOUS MOLLUSCA. 
the posterior dorsal accessory valve smooth and five-lobed : upper 
and lower edges with a horny-shelly epidermis. 1 ..2.— Panama. 
When young has no epidermis or accessory valve, but a large gape. 
P. Acuminata. Sow. Z. P.1834. Ovate, posteriorly rounded, 
anteriorly acuminated, with a very small gape; the valves divided 
into 3 surfaces, the posterior one roughish, the middle one with a 
horny epidermis, and transversely striated, the anterior one with 
smooth, gradually smaller, anteriorly acuminated, imbricated, horny 
scales: posterior ventral portion closed and smooth : posterior 
dorsal accessory valve, large, subquadrangular and anteriorly lobed : 
upper and lower edges with a shelly-horny epidermis, the dorsal 
covering posteriorly inflated. =%,;..2.--Panama. The young differ 
as in the last species. 
P. Mevanura. Sow. Z. P.1834. Ovate, posteriorly rounded, 
anteriorly obtuse, with a moderate gape; the valves divided by an 
impressed transversely grooved band ; the posterior area obliquely 
divided, the dorsal portion with radiating wrinkles, the ventral 
thinner and inflated ; anterior area concentrically striated, anteriorly 
with a black epidermis ; posterior dorsal edge inflated and reflected : 
with 2 anterior accessory valves which are sub-trigonal and vaulted 
above. 4..12.—W. Columbia. 
P. Cautirornica. Conrad. Jour. A. N.S. Phil. 7. p. 236. 
t. 18. f. 4.—An. P. Janeiui. Mag. de Zool. Elongated, poste- 
riorly rounded, anteriorly attenuated and subtruncated, ventral 
edge little arcuated ; posterior area gibbous, the concentric lines 
lamellar, crowded, undulated and crossed by numerous impressed 
lines ; accessory valves numerous, as well basal as dorsal: valves 
much compressed sub-medially, with an oblique groove, anterior 
slope angulated, anterior side with elevated laminz : apophysis 
oblique, dilated: hinge margin within greatly thickened anteriorly. 4. 
P. Quapra. Sow. Z. P. 1834. Oblong, very thin, posteriorly 
inflated, rounded, anteriorly sub-attenuated sub-truncated ; the valves 
divided by an impressed transversely grooved band: posterior area 
obliquely divided, the dorsal portion with concentric scaly lamellze, 
the ventral thinner, inflated, with radiating obsolete riblets : anterior 
surface with concentric grooves : posterior dorsal edge concave and 
reflected receiving the muscle, and covered with the epidermis which 
runs down upon four bladder-like substances, anteriorly, (two on 
each side) and then a calcareous tube conspicuous at the extremity. 
var.—The posterior dorsal edge inflated and reflected 53, ..1.—W. 
Columbia. 
P. Tuspirera. Sow. Z. P. 1834. Oblong, anteriorly sub- 
attenuated and sub-truncated, posteriorly rounded: valves divided 
by an impressed transversely grooved band: the posterior surface 
obliquely divided, the dorsal portion with radiating grooves, decus- 
sated, the ventral, thinner and sub-inflated: anterior area concen- 
