TRANSACTIONS OF WAGNER 

 1486 



TERTIARY FAUNA OF FLORIDA 



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Goodallia Turton, Dithyra Brit., p. 76, 1822. Type Mactra triangularis Montagu. 

 Mactrina Brown, 111. Conch. Gt. Brit., expl. pi. xvi., fig. 25, 1827 (= Mactra triangularis 



Montagu) = Goodallia Turton. 

 Microstagon Cossmann, Cat. Illustr. bassin de Paris, App. ii., p. n, 1896. No type cited. 



Ligament enfolding the resilium, both external on narrow nymphs. The 

 characteristics of this genus are set forth to a great extent in the remarks 

 under the head of Astartidce, and it will be reasonably clear to the reader that 

 a certain variability in characters usually stable is a marked feature of the 

 genus. After much study of recent species and the diagnoses of groups found 

 in the literature, I am of the opinion that these variations for the most part are 

 such as cannot be properly used for the subdivision of the genus. The crenu- 

 lation of the margin of the valves is not more than of specific value ; the obso- 

 lescence of the terminal cardinal teeth of the hinge, the greater or less promi- 

 nence of the lateral laminae, are characters which in this genus I have found 

 inconstant even in the species. The flattening of the umbones, which is so 

 conspicuous a character in many of the fossil species when a series of species 

 is studied, is seen to be so gradually modified between one species and another 

 as to admit of no hard and fast line being drawn between those with and those 

 without this character. By taking a single recent species to compare with 

 some fossil form it may chance that marked discrepancies may be noted, but 

 if a series of species be compared, these discrepancies will be found inconstant. 



The three earlier names, Astarte, Tridonta, and Crassina, practically cover 

 the same ground, the only difference being that Tridonta always has a smooth 

 inner margin while the fully developed type of the other groups has a crenu- 

 lated border. Why Leach should have given a new name to his two small 

 species is problematical ; one is sulcate, the other nearly smooth externally, 

 so it could not have been based on the sculpture. Both, however, were smaller 

 than the average British species and both had smooth margins. I have not 

 been able to examine specimens of Grotriania, but I have been unable to find 

 anything in the diagnoses or figures which authorizes us to separate it from 

 Astarte; Gonilia, Digitaria, and Rictocyma are sections frankly based on pecu- 

 liarities of external sculpture. Neocrassina is simply unusually inequilateral. 

 In Goodallia the hinge is reduced by the loss of anterior cardinals, but the 

 laterals still persist, though feeble. In its Eocene ancestor Microstagon one 

 of the lost cardinals can still be detected, though the laterals are inconstant 

 among the species. 



The genus may be divided, so far as its Tertiary and recent forms are con- 

 cerned, as follows : 



