54 OSWALD H. LATTER, M.A. 



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 Glochidium-s/ie^ to Shell of Adult. 



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

 deutscher Land- und Siisswasser-Mollusken, 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-sheW has a permanent 

 effect upon the shape of the adult shell. This effect will be at once 

 apparent on referring to Plate I. 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 Glochidium-sheW 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 otherwise 

 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 line 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 

 (i.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 the University Museum, Oxford, this 

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



