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771 



TERTIARY FAUNA OF FLORIDA ' ' 



very rare shell, the P. Cuiniiigii of west Central America. It is, in some 

 respects, intermediate between Epluppium and Monia Gray, but presents 

 additional characteristics of its own. 



The shell is strongly plicated with a few folds ; is attached when very 

 young, but may be free in the adult state. In the right valve the cardinal 

 margin is broad and strongly rugose with interlocking rugosities of both 

 valves. Though deep, they are too irregular in form to be called teeth. In 

 the right valve two .strong elevated crests — the auricular crura — meet above 

 at a very acute angle, and are received into sockets in the opposite valve, 

 separated by a space bearing a strong median ridge. The ligament connects 

 the outside of the crests with the sockets, but is continuous with a resilium 

 occupying the upper third of the space between the crests. The adductor 

 leaves a large subcentral, nearly circular, impression on both valves. The 

 byssal foramen is closed at an early age, leaving a round scar between that of 

 the adductor and the end of the anterior crest, which scar is joined to the 

 beak by a linear, solidly cemented suture. The byssal muscle persists as an 

 accessory adductor in function. There is no perforation of the shell nor 

 any necessary connection with external objects, in the adult state, any more 

 than in Epliippiuiii or Caiolia. None of the other genera of the group exhibit 

 the interlocking rugose cardinal area. 



The species by which the genus Placunanomia is usually judged belong 

 to a group, properly separated by Gray in 1849 (P. Z. S., p. 121) under the 

 name of Monia, with P. inacroschisma Deshayes as the type. In Monia there 

 is a very large, partly shelly, partly corneous, byssal plug, embraced by tJie 

 right valve (but with the suture always unsoldered, though close fitting), by 

 which the animal is attached at all stages of its existence, unless in the larval 

 condition. There is no cardinal area, no interlocking teeth or rugosities, or 

 paired, elevated internal crests or median internal resilium connecting the 

 valves below the chondrophoric arch. 



In Monia inacroschisma as compared with Placenta we have a condition 

 more like that of Epliippiuvi, but with a large notch and byssal plug, while 

 the chondrophoric margin is arched and not angulated, being represented by 

 a single pedunculated wide mass with a resilium under the arch. That por- 

 tion of the ligament attached to the chondrophore has become so large and 

 massive that it has supplanted entirely the remnant on the cardinal margin, 

 and the latter, at least in adults, is non-existent. In the left valve the margins 

 of the ligamentary scar are sometimes moderately thickened, but the process 



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