TRANSACTIONS OF WAGNER 

 770 



TERTIARY FAUNA OF FLORIDA 



ditioii. The hinge-margin in Ephippiimi is usually much worn and has lost 

 its original characters, but sometimes we find them preserved. They com- 

 prise an umbonal area covered with a thin ligamentary tissue and presenting, 

 under the umbo, an elevated subtriangular swelling recalling the deltidium in 

 some Brachiopods. The inner margins of the cardinal border in the right 

 valve are, as it were, detached, except at the umbonal end, and bent down- 

 ward into the cavity of the shell, carrying vi'ith them on their external edges 

 a portion of the ligament which, like the shelly crests upon which it is seated, 

 still remains continuous with the original ligament and cardinal border under 

 the umbo, though in the opposite (left or convex) valve it is mainly accom- 

 modated in a pair of grooves corresponding to the crests. In addition to this, 

 the ligament scnso stricto, there is also a small and feeble resilium situated in 

 the angle between the divaricating crests. Whether this is caused by an 

 advance with the growth of the shell of the external ligament over the angle 

 formed by the crests under the umbones, or is an original structure, the 

 material at my disposal is insufficient to decide. At all events, the sinus and 

 subsequent perforation is situated anteriorly to the anterior of the two chon- 

 drophores, or crests, as, according to the anatomical structure, is inevitable. 



In Placenta {P. placenta L.), however, we have a different state of things. 

 Here the cardinal margin is so narrow that the external ligament, if any, has 

 disappeared at an early age, leaving the two unequal chondrophores more 

 nearly parallel than occurs in Ephippium and not united in an angle at their 

 upper ends. The anterior cardinal margin is compressed into a narrow wing 

 with a groove for a byssus, as in some species of Pecten. The groove has 

 its edges reflected and thickened, and in most cabinet specimens, unless 

 eroded, is represented externally by a thread-like elevated ray. 



Pododesnms Philippi (Wiegm. Arch., 1837, P- 3^5) ^^'^^ founded on Placit- 

 nanoviia ntdis Brod. from Cuba, a species which differs from Monia Gray in 

 having a small, solidly soldered-up byssal foramen and a single muscular 

 impression {Jidc Philippi) ; the latter character would be true if the impres- 

 sion were regarded as additional to the byssal scar, but it is probable that 

 Philippi took the latter for the adductor scar and did not see the true muscu- 

 lar impression at all. However, it is likely that Monia, at best, can form no 

 more than a section of Pododesnms, which is long prior to Gray's subgenus. 

 From Carolia to Pododesnms is a short step ; another step, somewhat shorter, 

 brings us to Monia. 



The genus Placunanonda Broderip was founded on a remarkable and still 



