200 LAKES AND RIVERS. 



posterior side, mortised into the groove above men- 

 tioned. The length is i '5, the breadth 3. 



There are two varieties of this species. The first 

 has a green epidermis with yellow rays, the latter is 

 dark olive-brown. This species inhabits rivers, canals, 

 and ponds as far north as Yorkshire. It is one of 

 our Upper Tertiary fossils. Much larger specimens 

 occasionally occur. One is mentioned as having 

 been taken in Leicestershire, 4^ inches in breadth 

 and 2 inches long, the weight being above 3 ounces. 

 (Zoologist, 1857.) 



A single tumidus has been known to lay 1,500 eggs 

 in a few days ; they are deposited in clusters of about 

 100. U. pictorum has the body red, with a grayish 

 tint, the mantle bordered with brown, orifices like 

 the last described, the foot reddish or yellow-white, 

 tongue-shaped, the shell is long and compressed 

 underneath the epidermis ; in this and the last-named 

 species the shell is cream-white. The beaks are a little 

 incurved, and placed at a distance of between one- 

 fourth and one-fifth from the anterior side ; the um- 

 bonal region is not so prominent nor so strongly 

 wrinkled as in the preceding ; the teeth are finer, 

 sharper, and more erect, the muscular scars distinct, 

 the pallial scar faint, owing to the greater thickness 

 of the nacreous lining; the length is 1^33, the 

 breadth 3. The varieties are very numerous and 

 diverse, having peculiarities to which significant names 

 have been given. This species inhabits rivers, ponds, 

 and canals. 



Unto Margaritifera has the body dirty gray, with 

 sometimes a tint of flesh colour, the mantle bor- 



