56 MR. O. H. LATTER ON ANODON AND UNIO. [Jan. 20, 



entirely unable to make headway in any direction, even when they 

 are thus temporarily suspended in mid-water. 



The Glochidia are evidently peculiarly sensitive to the odour (?) 

 of fish. The tail of a recently killed Stickleback thrust into a 

 watch-glass containing Glochidia throws them all into the wildest 

 agitation for a few seconds ; the valves are violently closed and again 

 opened with astonishing rapidity for 15-25 seconds, and then the 

 animals appear exhausted and lie placid with widely gaping shells — 

 unless they chance to have closed upon any object in the water 

 (e. g. another Glochidium), in which case the valves remain firmly 

 closed. I found this excitement very useful in procuring Glochidia 

 widely open. Flooding with hot corrosive sublimate kills them 

 instantly and the shells remain apart. 



V. Relation of (aXochidixvivii- shell to Shell of Adult. 



So long ago as 1825 it was pointed out by Pfeiffer (Naturg. 

 deutscher Land- und Siisswasser-MoUusken, Weimar, 1825), and 

 more recently by Kobelt and Heynemann, that the shell of the 

 Glochidium sits like a saddle over the dorsal and lateral surfaces of 

 the shell of the young Anodon or may be seen in uninjured specimens 

 close to the hinge-line. It has not, however, been noticed, so far 

 as I can ascertain, that this temporary situation of the Glochidium- 

 shell has a permanent effect upon the shape of the adult shell. This 

 effect will be at once apparent on referring to Plate VII. figs. 2-5. 



About 101 days after first attachment to the host and 25-30 days 

 after quitting the host, the shell-teeth of the (rfoo/aWmj^-shell project 

 ventrally towards the median line, and as a consequence impinge 

 upon the ventral border of the at present soft shell of the adult at 

 a point about halfway along its length, the result being that at this 

 point the permanent shell is prevented from growing so fast as else- 

 where. The permanent shell at this stage, therefore, has its other- 

 wise symmetrical curve sharply interrupted by an irregular notch 

 pointing towards the dorsal surface {vide figs. 2 & 3). This notch, 

 in the vast majority of cases, persists through life and causes a slight 

 dorsal turn of the curves marking the lines of growth at a point 

 roughly halfway along their length, but, as a rule, slightly nearer 

 the posterior border of the shell. In each successive hue of the 

 growth the notch becomes of greater antero-posterior and less dorso- 

 ventral extent, thus tending to become less evident and to disappear. 

 The notch can therefore be seen most easily near the hinge-line 

 (^. e. on the first lines of growth) in those shells which have escaped 

 corrosion. In 15 species of Unio belonging to the Collection of 

 Admiral Sir John Harvey in tlie University Museum, Oxford, this 

 notch is evident and undoubtedly caused in the way above described ; 

 it is perhaps present in 2 others ( (I. cylindricus and U. triangularis), 

 and is quite clear also in 6 species of Anodon. The figures given 

 by Chenu in his ' Manuel de Conchyliologie,' and by M. Henri 

 Drouet, " Unionidse du Bassin du Rhone," Mem. de I'Acad. des Sci. 

 Arts et Belles-Lettres de Dijon, (4) i. 1888-89, pp. 27-113, 



