8 TESTACEOUS MOLLUSCA. 



tlie posterior dorsal accessory valve smooth and five-lobed : upper 

 and lower edges with a horny-shelly epidermis. 1 to • . 2. — Panama. 

 When young ha,s no epidermis or accessory valve, hut a large gape. 



P. Acuminata. Sow. Z. P.lQ^iA. 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. Melanura. 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 dorsa,l edge inflated and reflected : 

 with 2 anterior accessory valves which are sub-trigonal and vaulted 

 above. I . . 1 §. — W. Columbia. 



P. Californica. Conrad. Jour. A. N. S. Phil. 1. p. 2SG. 

 t. IS./". 4. — An. P. Janelli. Mag. de Zoo/, 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 laminae : apophysis 

 oblique, dilated: hinge margin within greatly thickened anteriorly. 4. 



P. Quadra. So7v. 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 lamellae, 

 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-l^ke substances, anteriorly, (two on 

 each side) and then a calcareous tube conspicuous at the extremity. 

 var. — The posterior dorsal edge inflated and reflected -jS^y . . I . — W. 

 Columbia. 



P. TuBiFERA. 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- 



