112 MYA. 



less in breadth. Young shells are smaller, of a purple 

 colour within, with prominent wrinkles diverging from 

 the apex, nearly to the margin. 



Dr. Maton describes this shell as varying so ex- 

 tremely in its structure and contour, according to its 

 several stages of growth, that if he had not been able 

 to compare them in their intermediate ages, he should 

 probably have been led to consider them as distinct 

 species ; a mistake which bas often originated, as the 

 Doctor very justly remarks, from the want of similar 

 opportunities. 



It is a fresh-water shell, and inhabits the Rio de la 

 Plata, in South America. 



LIPPED GAPER. 



39. Mya Iabiata. M. testa subovali, valvis occlusissimis, alterius mar- 

 gine labii instar prominente. Linn. Trans. 10. p. 326. t. 24. f. 1, 2, 3. 

 Shell suboval, valves shut close, with one margin projecting like a lip. 



A strong shell, with a green epidermis, transversely 

 striated, the fore part is somewhat beaked ; the inside 

 is pearly and shining. The hinge has a solid, projecting 

 tooth, in one valve, which is rather spoon-shaped, and 

 fits into a triangular cavity in the opposite valve ; the 

 umbones are very slightly prominent. 



This shell described with the above by Dr. Maton, 

 was also received, through the medium of the Right 

 Hon. Sir Joseph Banks, from the asstuary of the Rio 

 de la Plata. The specimen from which the characters 

 were taken, measures about an inch in length, and 

 rather more than half an inch in width. Tts most 

 striking characteristic is the inequality of its valves, 

 part of the margin of one valve, projecting over the 

 corresponding part of the other, so as, exactly to re- 



