MiDuJiestcr McDioirs, Vol. /v. (igii), No. H. 5 



are not eroded at the beaks. This last is because the 

 h'mestone districts are unfavourable for the production of 

 bogs, and the banks of chalk and limestone streams are 

 not covered so thickly with vegetation as those of streams 

 in clay districts, so that the streams in chalk and limestone 

 districts are not charged with humic acid and the shells in 

 them are not eroded. 



The greater thickness of the shell and animal in the 

 Keuper districts seems to be connected either with the 

 presence of humic acid, or the absence of excess of chalk. 



4. Anodonta cygnca shows a remarkable change of 

 form during growth. The once so-called species A. aiiatina 

 has been definitely shown to be merely a growth stage of 

 A. cygnea. 



The glochidral shell (= phylembryonic stage of 

 Aviculidai. — R. T. Jackson) of this animal is exceedingly 

 thin and has a perfectly straight hinge line when the valves 

 are shut. The nepionic [umbonal] markings are less 

 pronounced than in the two Uniones studied, and consist 

 of more or less concentric ridges, varying in number and 

 thickness. 



From the commencement of the nepionic stage the 

 young animal ploughs its way through the mud, with the 

 result that growth at the anterior end is hindered, so 

 the posterior end develops more rapidly. The unequal 

 growth is more marked than in the typical Uniones, 

 probably because the shell is far thinner throughout life in 

 A nodon. 



At the end of the third year the posterior end has so 

 gained in growth on the anterior that the originally central 

 position of the umbo has been lost, the post-umbonal region 

 being longer than the pre-umbonal. 



Anodons, when moving through the mud under the 

 water, plough along with the head directed downwards, so 



