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146^ 



TERTIARY FAUNA OF FLORIDA h J 



scription Conrad founded his genus Scambula in 1869. Crassatellina of Meek, 

 referred to this family by its author in 1871, does not belong here; it is a 

 form closely allied to Cyclas Link, from which it and the synonymous Etea 

 of Conrad differ by possessing lateral teeth and a more elongate and angular 

 form. Ptychomya Agassiz is distinguished especially by the divaricate ex- 

 ternal sculpture (I have not been able to study the hinge, which is inaccurately 

 described in the manuals) and is referred by Fischer to the Venerida. Cras- 

 sitina Weinkauff, 1881, was proposed for the smaller recent species, which 

 resemble Pachythcerus except in the greater development of the resiliary pit. 



The development and homologies of the hinge-teeth in Crassatellites pre- 

 sent some difficulties. Disregarding the so-called internal position of the liga- 

 ment common to most bivalves in the larval state, there can be no doubt that 

 in the ancestors of this group the ligament and probably the resilium were 

 external and, from the upper Cretaceous to the present day, the tendency of 

 these organs has been to become more and more internal until both of them 

 have no trace of externality. They have, in short, migrated ventrad. 



According to the work of the lamented F. Bernard,* C. plumbeus and C. 

 lamellosus exhibit in early stages of growth the following formula for the 



L. oio.oioi.oio i Now in all my study of the hinges of our American fossil 



R. lol.ioio.lol 



species and a considerable number of recent forms I have not found a formula 

 of this kind. I have not been able to examine specimens less than three milli- 

 metres in diameter, but if the formula is as above even these should, I suppose, 

 give evidence of it. The letter / in the above formula stands for lateral laminae, 

 and the ciphers for gaps or sockets into which laminae or teeth fit. 



According to my observations on American species the formula should be 

 L ioi.oioio.oio _ That i s to say, the right valve has always three cardinals, of which 



R. olo.ioioi.lol 



the posterior one is more or less smothered and obliterated by the descending 

 resilium, while the left valve has only two cardinals. Furthermore, there are 

 nowhere any developed laterals at all. What I have represented in the above 

 formula by / may or may not be obsolete laterals, but, whatever view be taken 

 of them, the facts are as follows : In the right valve the posterior hinge-margin 

 is simply bevelled and is received into a groove in the corresponding part of 

 the opposite valve ; the right anterior hinge-margin has a shorter groove into 

 which the bevelled edge of the left valve is received. There are no differ- 

 entiated laminae in the species I have examined. But in the little Crassinella 

 (mactracea] or Pseuderiphyla the lower margin of each of these grooves be- 



*Bull. Soc. Geol. de France, 3me Sen, xxiii., p. 121, 1895. 



