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827 



TERTIARY FAUNA OF FLORIDA 



The little irregularities, especially notable near the anterior end, are probably 

 due to attached particles of gravel on the original tube. 



FAMILY SAXICAVID/E. 

 r.emis PANOPEA Mcnurd. 



l\infif>i-ii Mi-nard, Annales clu Mtis. Paris, ix., p. 135, 1807 ; doldf., Hanclb. d. Zool., p. 



677, 1820. 

 (I'/yi'init'i'fs Lamarck, Prodrome, p. 83, 1799. Type Mya gfycytneris l!orn. (Not Glyeymeru 



Da Costa, 1778, nor (//>'< /wrvv'v Lain., iSoi, nor Schumacher, 1817.) 

 /\in<>/>u~ti Lain., Kxtr. d'un Cours., p. 108, 1812; An. s. Vert., v., 11.456, 1818; Valenci- 



cnnes, Arch, du Mus. Paris, i, p. 3, 1838. 

 I'anofna Swainson, Malac., p. 367, 1840. 

 lilycimcris H. and A. Adams, den. Rec. Moll., ii., p. 350, 1856; Gray, Fig. Moll. An., 



v., p. 30, 1857. 



This well-known genus, after the exclusion of the Saxicavoid species, 

 forms a very natural group, related to the Myadd<c on the one hand and to 

 Sti.ricava on the other. Some pearly forms formerly confounded with it have 

 long been eliminated, and have relations, no doubt, with Anatinacca. 



I have had the advantage of an opportunity to study several Pacific 

 coast forms in life and in their natural surroundings, as well as a very large 

 series of our Tertiary species, and also a fair series of most of the recent 

 exotic species. For that reason, perhaps, the following conclusions will have 

 a certain value, which is only derived from a somewhat extended range of 

 observation of the animals themselves. 



All boring mollusks in which the shell has so degenerated that it no 

 longer covers the whole adult animal when retracted are more liable to varia- 

 tion in minor details than those in which the valves meet distally, and dynami- 

 cally influence their own development by fixing for it certain definite limits. 

 This is markedly the case in the present genus. Those shells which live in 

 an easily movable medium, such as sand or fine, soft mud, arc thinner, better 

 developed, more elongated, and less distorted than their congeners who are 

 obliged to confine themselves to a gravelly or stony situs. So marked is the 

 difference that I have several times been presented with supposed new species 

 based on these dynamic characters, and, by a curious reversal of logic, have 

 been assured that the differences must be specific, because the animals in- 

 habited, respectively, the different kinds of ground alluded to. 



I have observed, also, that where the ground into which the bummers 



O 



