TRANSACTIONS OF WAGNER 

 870 



' TERTIARY FAUNA OF FLORIDA 



fossils, such as Alactropsis Conrad, must be governed by the general habit of 

 the shell, since no other means is left us to decide by. 



Mactra is, on the whole, a somewhat active animal, and seems to prefer 

 clean sand, through which it ploughs at times with its strong Cardimn-XxVQ foot, 

 leaving long furrows behind it. Certain forms seem to favor a sedentary life, 

 many of the Mulinias and Raiigia, and in these the foot has become smaller, 

 and the hinge more amorphous in appearance. In a still more modified 

 division, which has, to all intents and purposes, become absolutely sedentary, 

 and which inhabits deeper water than the above-mentioned Mulinias, the 

 dynamic modifications characteristic of this class of vertical borers have been 

 more or less fully adopted. The body has become elongated, the siphons 

 lengthened, the epidermal sheath necessary for protection to the permanently 

 extruded siphons is continued to their ends, the mantle has become soldered 

 ventrally as closely as the use of the foot for boring will permit, the shell has 

 become more asymmetrical relative to the hinge. The moUusk, which has 

 really made of its permanent tunnel an artificial shell, does not materially suffer 

 by the exposure within that tunnel of a greater proportion of shell-less surface, 

 and from the gradual degradation of a hinge which has become no longer of 

 vital importance. All these modifications are of tlie kind I class as " dy- 

 namical," and though, by heredity, they may eventually be permanently im- 

 pressed on the organism, yet it will hardly be claimed that they have as great 

 a value for the higher systematic divisions .as those characters which, derived 

 from unknown antiquity, we are able to recognize as genetic. Yet it is found 

 in one of the latest morphological essays at a systematic arrangement of the 

 Pelecypods that Ltttraria is put in a different Order from Mactra because of 

 the closure of the ventral opening of the mantle in part of the species ! Such 

 characters are common to forms as fundamentally divergent as Solemya, Solm, 

 Glycimeris, Tagelus, Mya, and Cyrtodaria, and indicate nothing more than a 

 common (and inevitable) response to common dynamic conditions of life. 

 Such conditions have existed, it is true, from the beginning, and Solemya is 

 witness to a very ancient response; nevertheless, there is nothing in any of 

 the mutations which is indicated as a permanent necessary part of the or- 

 ganism, or which might not pass away with ease under a prolonged change 

 of conditions. 



Before proceeding to discuss the various groups of Mactracca, it is de- 

 sirable to refer more particularly to the views of the late Professor Neumayr, 

 to whom we owe such a stimulating and important discussion of the morph- 



