FREE INSTITUTE OF SCIENCE 



TERTIARY FAUNA OF FLORIDA 



Mr. E. A. Smith announced the discovery by the Challenger Expedition of 

 /'. iiTa/is at the Cape of Good Hope, in fifteen to twenty fathoms, at St. Simon's 

 Bay. I have not examined these specimens. 



The typical species of Plcnrodon is characterized by a shell externally 

 resembling Nnciila, but having a structure much less nacreous. Mr. Wood, 

 indeed, described his shell as nacreous, but none of the specimens given by 

 him to Dr. Jeffreys have the least pearly lustre, and if the recent species are 

 examined they appear hardly more nacreous than an ordinary Leda. The 

 anterior side of the hinge-line is short, the cardinal border externally is pro- 

 duced and angulated, and in an excavation under this little angle lies the 

 ligament which in a normal and perfectly preserved specimen is nearly or 

 quite covered by the margin of the valves. In most specimens this covering, 

 being extremely thin, is eroded or broken away, so that two valves in oppo- 

 sition show a small oval pocket in which the ligament was originally con- 

 tained. In /*. miliaris Deshaycs the perfect shell completely hides the ligament, 

 which is wholly internal, and the British specimens indicate that this may 

 have been the case with P. m-alis also. In both of these species and in the 

 recent /'. Adatnsii from Florida as well as the P. Woodii from the Caloosa- 

 hatchie, the cavity for the ligament is rather flattish and small ; in the P. 

 iniiiiita Cpr. from California as well as the Japanese Cyrilla the cavity is large 

 and nearly spherical. 



The cardinal plate in all the species is rather broad, and terminates in the 

 left valve with a prominent lateral tooth which is received into a corresponding 

 depressiop in the plate of the opposite valve, the edge of the plate in most of 

 the species being turned up like a tooth, but in P. inn nit a remaining flat in the 

 right valve. The cardinal teeth between the ligamentary fossette and the 

 lateral tooth or socket vary in form and arrangement in each species. They 

 are, from their minuteness and complexity, very difficult objects to observe 

 and draw correctly. They are set in a sort of arch, under the beak, but a 

 careful and minute study shows that the series is really composed of two dis- 

 tinct groups, one belonging to the anterior and one to the posterior side of 

 the hinge, and the teeth in one group are usually of a different type from those 

 in the other group. The two rows or groups approach one another at a 

 more or less evident angle, recalling the two groups in Nncitla : while a 

 reminiscence of the internal fossette of Nncnla remains in a slight broadening 

 of the cardinal plate just below this angle. The edge in P. Woodii is actually 

 produced into a little angular projection here, but the recent species do not 



