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TERTIARY FAUNA OF FLORIDA 



corbula does to Cuneocorbula, or Here to Lucina. In Philis the lunular region 

 is deeply impressed to make a subspherical fossette, as in the other groups 

 referred to. It is noticeable that in all the cases where this peculiar feature 

 occurs thejjyxidiferous form is tropical, while the related forms, without the 

 fossette, are from cooler water. 



The largest known species is the Thyasira bisecta of Conrad from the Mio- 

 cene of the Pacific coast, which has survived in the deeper water of Puget 

 Sound until the present day. It has occurred to me that possibly the anomalous 

 genus Ludovicia (Desh. MS.) Cossmann may be related to Thyasira (cf. 

 Cossm., 111. Cat., ii., p. 49, pi. ii., figs. 21-22, 1887). Some of the elongated 

 species, if the posterior fold were smoothed 'out (and it is sometimes nearly 

 obsolete), would much resemble Ludovicia squamula. 



The most prominent features of the larger species of Thyasira comprise the 

 posterior plications of the shell, the indentation of the hinge-margin just in 

 front of the beaks, which forms a pseudo-tooth, as long since observed by 

 Philippi and others, the edentulous hinge-margin, lucinoid adductor scars, and 

 thin, colorless valves with entire margins. The plications in the very young 

 and in the smaller species are frequently obsolete or absent, but every stage 

 between those with and those without them is observable in a large collection 

 of species. While we may separate the smooth forms for convenience section- 

 ally, there will be some which will only be assigned a place with difficulty, being 

 so strictly intermediate. The species follow in many respects the mutations 

 of the Lucinas. Both have a minute, deeply impressed lunule in many forms, 

 which is more patulous in the right valve than in the other, and often grades 

 into a dentiform process fitting into a socket in the valve opposite. In the 

 various species of Axinopsis this formation may be traced through mutations 

 which culminate in a well-developed tooth in the most specialized species. In 

 both Lucina (senso lato) and Thyasira the anterior and posterior dorsal areas 

 (or plications) grade from most pronounced formations to obsolescence. In 

 both the ligulate anterior adductor scar varies from long to short. In both the 

 ligament and resilium are generally united and inset in a groove, though in 

 Thyasira the hinge-margin is so thin that there is no opportunity for the resilium 

 to separate from the ligament and become embedded by itself in the mass of the 

 hinge-plate, as occurs in Loripes. The only change I have observed corre- 

 sponding to this is a relative thickening of the resilium, which becomes spread 

 out on the under side of the hinge-margin without, however, becoming truly 

 internal, i.e., invisible from outside when the valves are closed. 



In Thyasira proper I have seen no instance in which a true tooth is de- 



