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691 



TERTIARY FAUNA OF FLORIDA 



by Conrad the " submargins." Above the submargins the auricles, or ears, 

 project, usually differentiated by a linear depression ending in a sinus below. 

 This sinus is sometimes absent in the posterior ears, as it is in the very young 

 stages of tlie shell, but it is not an important systematic character, since the 

 same species (<. </-., /'. latiauritus Conr.) may exhibit varieties some of which 

 have a well-developed posterior sinus while others are without it. The right 

 anterior sinus is usually emphasized by a flexuosity in the lower edge of the 

 ear above it for the accommodation of the byssus, and on the upper part of 

 the submargin are usually found a number of small, regularly spaced spines, 

 which in life separate the threads of the byssus and thus keep it from twisting 

 with the motion of the water. The growth of the margin of the valve and 

 car does not always march with the development of these spines, so that a 

 species which normally has them may exhibit stages when the valve margin 

 has grown over the old set and the new set has not been formed, much like 

 the inequalities of growth shown by the margin of the aperture and the 

 internal lira; of some Gastropods. This set of spines, resembling a short 

 comb with curved teeth, has been called ctcnolium, pectineum, and pectin- 

 iclium. In old very heavy shells, which are held in place more by their own 

 weight than by the formation of a byssus, they are often absent, but may 

 usually be traced in the groove corresponding to the younger stages, or 

 fascicle, of the sinus. 



The swimming habit of Pec ten is well known. It is more commonly 

 exercised by the thin-shelled light or young individuals than by the heavier 

 or adult specimens. The lateral ends of the ears do not close tightly. The 

 valves being open, a quantity of water is retained between the inner lamina? 

 or " curtains" of the mantle, and the contraction of the adductor forces this 

 water out between the submargins and through the cavity of the auricles, 

 which impels the animal forward, the ventral margin of the valves being in 

 advance as it moves. In this way the Pcelen moves quite rapidly with a 

 jerking motion. 



In proportion to its surface the shell of Pec ten is thin, and in the adult is 

 usually ribbed or fluted, a condition brought about, doubtless, by natural 

 selection and serving to strengthen the valves, which in swimming and falling 

 on the bottom arc subjected to rude shocks. When these flirtings are formed 

 in a thin shell, the interior is usually grooved in harmony with the ribbing of 

 the exterior. To still further strengthen the shell at its weakest point, when 

 the flirtings are of angular section, a linear deposit of shelly matter is often 



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