FREE INSTITUTE OF SCIENCE , 



1397 



TERTIARY FAUNA OF FLORIDA 



and hence Da Costa and some of the other early writers objected to the Linnean 

 use of it for a genus which closes very tightly, but the objection has not been 

 sustained. The type of Chama Da Costa is Mya truncata. 



As is natural, owing to their variability and the fact that the spines or 

 lamellse may in individuals become obsolete, the number of species of Chama 

 seems to have been exaggerated by authors. On the other hand, these very 

 factors render it more difficult to discriminate between nearly allied species 

 which may really deserve separation. 



A group based on the nepionic young of Chama has been named Goossensia 

 by Cossmann and appropriately placed in the Carditidce, of which I believe our 

 modern Chamidce to be an offshoot. I have not the material suitable to base 

 an opinion upon as to the origin of the early Mesozoic forms which have been 

 referred to this family, but should not be surprised if a sufficiently thorough 

 study showed that they are less intimately connected with the Tertiary and 

 recent forms than has been supposed, the characteristics upon which their 

 alliance has been assumed being dynamic rather than genetic. It is not im- 

 probably a case similar to that of Hinnites, in which the forms existing in 

 different epochs have diverged sporadically from the contemporaneous Pec- 

 tinidcE and have no direct genetic connection from one horizon to another. 



The mutations within the species of Chama are quite marked. They com- 

 prise color variations which are often quite striking, as lemon-yellow and pale 

 or dark purple in C. macerophylla, profuse, sparse, or obsolete foliation, and 

 such changes of form as are due to the object upon which they are fixed. In 

 discriminating species these fluctuations should be taken into account by the 

 student, but it will also be found that there are features which are tolerably 

 constant and which, after due discrimination, will be found to serve as guides 

 to specific identity. 



Chama mississippiensis Conrad. 

 Chama mississippiensis Conrad, Proc. Acad. Nat. Sci. Phila., iii., p. 294, 1847; Journ. 



Acad. Nat. Sci. Phila., 2d Ser., i., p. 124, pi. xiii., figs. 21, 27, 1848; Checkl. Eoc. Fos. 



N. Am., p. 27, 1866; Am. Journ. Conch., i., p. 7, 1865. 



Vicksburgian Oligocene at Vicksburg and at Red Bluff and other localities 

 in Wayne County, Mississippi ; in Louisiana, near Mt. Lebanon ; Vaughan. 



This is quite a small species attached by the right valve and with low, sparse, 

 pustular spines. 



An Eocene species from the upper Midway limestone has been described by 

 Harris (Bull. Pal., i., p. 180, pi. vi., figs. 4, 4a, 1896) under the name of C. 



