720 SYSTEMATIC PALEONTOLOGY 



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 observations 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 

 variation in minute details than those in which the valves meet distally, 

 and dynamically 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, are thinner, better developed, more elongated and less distorted than 

 their cogeners who are obliged to confine themselves to a gravelly or stony 

 situs. So marked is the difference that 1 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 inhabited, respectively, the different kinds 

 of ground alluded to. 



" I have observed, also, that where the ground into which the burrowers 

 retire is a comparatively thin coating over a stony or rocky layer which 

 they cannot pierce, the tendency in Panopea, Mya, etc., is for relatively 

 short and broad shells, with shorter siphons, to survive ; which naturally 

 have a wider, shorter, and more rounded pallial sinus and shorter and 

 more incurved nymphs. I believe the influence of the environment is 

 direct and not selective; at all events, the association of situs and speci- 

 mens so characterized is, as far as I have been able to determine, quite 

 uniform, whether selective or not 



" In addition to the differences more or less evidently due to situs there 

 is a series of differences which occur among specimens of a single species 

 from apparently the same situs, both in the fossil and recent forms. These 

 include a nearly rectilinear as compared with an arcuate hinge line, and a 

 short as opposed to a long insertion of the ligament. The length of the 

 ligament is perhaps co-ordinated with the heaviness of the valves, but the 

 differences alluded to occur so constantly that I have been led to suspect 



