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TERTIARY FAUNA OF FLORIDA 



haps by a lapsus pennce. But in most cases there is no duplication, and in 

 many species the laterals have entirely vanished. 



The ligament and resilium are usually, though not always, united in this 

 family, generally somewhat inset, but rarely truly internal. In Codakia may be 

 noted (in fresh and still united specimens) a unique feature in that a solid 

 shelly coating is deposited on the external surface of the ligament. It is usually 

 scaled off in dry specimens if an attempt to separate the valves has been made. 



The valves of the more characteristic Lucinida: have usually two more or 

 less compressed areas dorsally, marked off by this compression, by a radial 

 sulcus, or by a change in sculpture from that in the middle part of the disk. 

 These I have termed the anterior and posterior dorsal areas ; they are some- 

 times spoken of as anal and oral areas. They are sometimes obsolete or, espe- 

 cially the anterior one, marked off only by an impressed line. This may be 

 mistaken for a lunular boundary ; but the lunule, which in this group is usually 

 small and often asymmetrically developed as regards the two valves, is usually 

 demarcated by a ridge or angle, and not, as in Veneridce, by an incised line. 

 The lunular characters are very constant and useful in this group for specific 

 discrimination, such variation as there is in them being chiefly in the matter of 

 depth below the general level of the valve. In general the concentric sculpture 

 dominates the radial ; I have seen no species in which radial sculpture is present 

 to the exclusion of the concentric. One might suppose that external sculpture, 

 being one of the characters most subject to be impressed by environmental 

 conditions, would be inconstant compared with some of the less exposed features. 

 This, however, is not the case ; the types of sculpture are remarkably persistent, 

 existing without essential change from the earliest Tertiary in many lines of 

 Lucinoid development. The explanation probably is that the varieties of con- 

 ditions which affect sculpture are limited in number and very constant from 

 one geological era to another, and certain types, having developed a sculpture 

 which is qualified to harmonize with these conditions, hand it on to their de- 

 scendants practically without change from age to age. The margins of the 

 valves are usually entire or feebly crenulate, in conformity with the absence 

 of strong radial sculpture. The pallial line is always entire, but often rather 

 rude or ragged. The anterior adductor scars afford one of the most charac- 

 teristic Lucinoid characters in their tendency to a prolongation within the disk 

 bounded by the pallial line. They sometimes reach nearly halfway across the 

 disk and extend below the centre of mass of the body. The posterior scar is 

 more rounded and higher up in the valve, but it should be noted that the amount 

 of prolongation of the anterior scar appears to vary very considerably in allied 



