35 



centric wrinkles; umbo placed near to the larger end; 

 hinge furnilhed with one tooth, which locks into a bifur- 

 cated tooth in the oppofite valve; thefe are flrong and 

 crenated ; befides which it has lateral lamina^ two in one 

 valve, and one in the other. Infide pearlaceous ; Length 

 fometimes near two inches; breadth four, A varietv is 

 frequently found of an olive-green, thin and fciniprllucid, 

 with a few faint radiated (Irire from the umbo : the teeth 

 not near fo ftrong, and the infide refplendent like inother- 

 of-pearl. 



This variety has been confidered by fome as a diftinft 

 fpecies : but as they are to be found in all gradations in 

 lubftance and colour, fuch muft be confidered as varieties 

 only. The thin fliells, and thofe which are covered with 

 a green or yellowifli epidermis, are mod commonly found 

 in ponds; whereas the dark and ftrong (hells inhabit rivers. 



This fpecies has been defcribed and figured by many 

 authors for the M. Pidorum of Linn.eus. Doctor Pljl- 

 TENEY confiders it perfetlly diftinft from that fliell, and 

 indeed we cannot but aftent to his opinion. It is mofl 

 probable Linn^us was not acquainted with this fpecies : 

 and Gmelin has certainly confounded the two, if we 

 may judge from his fynonyms, for he has quoted both 

 Lister and Chemnitz for the Pi&orum, whereas the 

 former has not given a figure of that Ihell, but evidently 

 two varieties of this ; the latter has figured the true 

 Pidonim. 



F 2 The 



