266 ME. A. R. HUNT 0]S THE INFLTJENCE OF 



tide-marhs find their safety in their power of maintaining them- 

 selves to a greater or less extent below the surface of such 

 bottom. This position of safety is attained in two ways, viz, 

 either by the power of the animal to burrow rapidly in the deposit 

 in which it lives, or by its power of retaining a safe position 

 when once secured. Mr. Osier, referring to the burrowing of 

 bivalves, has observed that " animals which from their small size 

 or the little depth at which they reside are particularly liable to 

 be exposed, will generally burrow readily ; but the large species 

 will scarcely attempt to bury themselves, except in the very young 

 state " (Phil. Trans. 1826, p. 348). Irrespective of age, activity 

 in burrowing varies greatly in different instances. This can be 

 well seen in the case of the young of different species of Qardium, 

 and in that of full-grown specimens of Psammohia tellinella. The 

 compressed valves and hatchet-like form of the latter are clearly 

 suited for rapid penetration, whereas the comparatively globular 

 form of the Cardium is as evidently ill adapted to force its way 

 through a resisting medium. As a matter of fact, when in capti- 

 vity the cockles burrow very sluggishly, if at all, whereas the Psatn- 

 mobias do so most readily. In their natural homes the latter 

 seek to escape by evading the wave-currents, the former to a great 

 extent by passive resistance. JPsammoiia tellinella abounds in a 

 deposit consisting of comminuted shells and small stones which 

 occurs in the vicinity of Hope's Nose, the northern horn of Tor- 

 bay, the materials being accumulated by strong currents. Through 

 this deposit the Psammohia forces its way with great ease. If it 

 were otherwise it is difficult to see how it could survive, as the 

 character of the accumulation is sufficient proof of the amount of 

 disturbance to which it is occasionally exposed. 



The large cockle, Oardium aculeatum, is abundant in fine 

 muddy sand off Paignton in Torbay. In this deposit the long 

 spines of this species are of service to increase its holding-powers. 

 An allied species, C. tuherculatum., whose shell is rough and 

 ribbed but not spined, dwells in hard sand at and below low-water 

 mark off the same place. The two species are not commonly found 

 living together, though I have found specimens of C. aculeatum 

 that had been washed in from sea, and their spines denuded in 

 the process, living side by side in the hard ground with G. tuher- 

 culatum. In both these species the rough shells tend to prevent 

 the moUusks being readily dislodged by the waves, Each species 

 keeps to the ground best suited to it. The spines of O. aculeatvm 



