"4 



SCIENCE-GOSSIP. 



EROSION IX EXTRA-MARINE ^lOLLUSCA. 

 By Arthur E. Boycott. 



Q OME years ago I collected some 1,100 or 1,200 

 Unio pktorum and Anodonia cygnaea in some 

 ponds in Kent. The ponds were apparently a 

 series of old marl pits, and in them these two 

 species occurred in great profusion, so much so 

 that after a successful day's work in the water, 

 sacks were necessar\- to take home the spoil. 

 Variation in the long series thus procured vras not 

 unnaturally frequent, but at present I desire to 

 draw attention to a certain form of erosion which I 

 noticed in the specimens from this locality. Both 

 Unio and Anodonta were a good deal eroded, but 

 not excessively so, and in some of the ponds 

 Bithinia tentaculata {}) and B. leachii also occurred, 

 much eroded and decollated, the decollation here 

 being, of course, only a special form of erosion. 

 Besides the usual absence of periostracum round 

 the umbones, many specimens, especially of U. 

 pktorum, exhibited a curious kind of erosion else- 

 where on the shell, very commonly near the ventral 

 border. It consists of comparatively deep and 

 narrow pits and grooves, the latter running approxi- 

 mately parallel -with the lines of growth. Now 

 this pit erosion in not very rare, but the noticeable 

 point about it is that in almost all cases a 

 very nearh- similar erosion was present on both 

 valves. Whatever was the cause of the malforma- 

 tion, the mollusc seems to have had a strong desire 

 to make its shell as nearly bilaterally symmetrical 

 as possible. The shells mostly occurred at the time 

 of collecting (September), with their anterior two- 

 thirds buried in the mud, and of course the cause 

 of the similarity of the erosion on the two valves 

 may have been the fact that the two sides were 

 almost precisely equally exposed to or protected 

 from the mud or water. I may mention that the 

 pits and grooves occur both at the anterior and 

 posterior ends, if anything more frequently at 

 the latter. Since first noticing this symmetrical 

 erosion, I have come across several similar cases. 

 For instance, I have a specimen of Sphaerium 

 corneum on a caddis-case, which was e\-idently put 

 under contribution when alive, as the valve, which 

 is stuck on to the case, is stronglj' and distinctly 

 indented at the point of attachment ; this indenta- 

 tion is reproduced on the free valve. Again, in 

 examining a number of young A. cygnaea, I found 

 that many cases of slight malformations from 

 injuries had been reproduced, so that both valves 

 were very similar in appearance. It is easy to 



(') I may add that in these extensively eroded B. fentaculata 

 the following points struck me : the shells were thicker, and 

 the opercula were thicker, more concave, with stronger 

 striae, and appeared larger in comparison with the mouth of 

 the shell. 



imagine how the edge of the mantle on both sides 

 might be injured simultaneously, e.g., by a fish 

 biting at the shell, but it is not very clear how 

 these young Anodonta could have been injured at 

 all in this wa}-. They occurred in the soft mud at 

 the bottom of a large artificial tank, where there 

 were a few pike and perch, and numbers of roach, 

 but I noticed that the shells crawled about nearly 

 under the surface of the mud, the j'oung ones 

 going especially deep. At any rate, the symmetry 

 of the injury, however caused, was very apparent 

 in man}' cases. It is noticeable, in passing, that 

 though the mud at the bottom was rather foul, yet 

 the shells of both A . cygnaea and U. pictonnn and 

 tumidus there occurring were remarkably clean and 

 bright, owing, no doubt, to the fact that the water 

 is always fresh and clean. 



The same kind of symmetry is often obser\-able 

 in the erosion round the umbones of Unionidae, but 

 it is, as a rule, I think, much less exact than in 

 many of the pit and groove cases. Where the 

 ordinary patch erosion runs very deep I have not, 

 as a rule, noticed much correspondence between 

 the valves. Near Hereford, on the Wye, I once 

 took some Margaritana margaritifera, in which the 

 erosion had apparently gone right through the shell. 

 Sand had entered the hole, but it had soon been 

 skinned over by the animal, and eventuallj' a hard 

 lump — often of considerable size — of agglutinated 

 sand was formed \\'ithin the shell. I have not seen 

 a specimen in which this erosion had gone right 

 through both valves. 



I find that J. W. Ta5-lor(2) has figured a very 

 good case of symmetrical injurj- in A. cygnaea, 

 which he attributes to some severe injury of the 

 mantle. As in most cases of shell-distortion which 

 are appro.ximately symmetrical, one side is worse 

 than the other. 



The explanation of the causes of erosion in fresh- 

 water shells seems almost as far distant as ever. 

 That is to say, authorities do not seem to agree as 

 to the reason, or reasons, for the initial loss of 

 the periostracum. That CO2 in solution, as it is 

 in almost every water, will dissolve away the 

 CaCOg of the shell seems certain enough, but the 

 loss of the organic covering of the shell cannot, 

 apparently, be due to the same cause. The 

 explanations to account for the wearing of the 

 periostracum are verj- numerous : some sa}' it is 

 caused by the boring of microscopic fungi (") , and 

 it has often been stated that other snails eat away 



(') Monograph i. (1S95), p. 103, fig. 216. 



(') On this subject see E. Bornet and C. Flahault, Bull. 

 Soc. Hot. de France, xxxvi. (1890), pp. 147-176; Abstract in 

 "Nature," xliii., p. 1S5. 



