AND OTHER FAMILIES. 79 



Dlam, 1-5, Length 1-9, Breadth 3-3 inches. 



Shell elliptical, transverse, inequilateral, with a broad horn coloured 

 border, emarginate at base ; sinus incurved ; substance of the shell thick : 

 beaks scarcely prominent; ligament long and thick; epidermis dark 

 olive brown, wrinkled, obscurely rayed on the posterior slope : anterior 

 cicatrices distinct; posterior cicatrices confluent; palleal cicatrix large 

 and partially tinted with bluish purple; dorsal cicatrix situated in the 

 centre of the cavity of the beaks ; cavity of the beaks very shallow : 

 nacre pearly white clouded with bluish purple, extending only to the 

 broad horn coloured border, iridescent. 



Memarks. — This curious species is from the collection sent to the 

 Academy of Natural Sciences by Dr Burrough. It difTers distinctly 

 from any species known to me. The horn coloured broad border, and 

 the absence of nacreous matter on this part is very remarkable, as is 

 also the close approximation to a perfect ellipsis, the posterior and an- 

 terior margins being nearly of the same curve. The clouded bluish 

 purple colour I have never seen in the nacre of any other species. The 

 sinus is so peculiar in the two specimens examined, that I would 

 impress it as important in the character of this species. In the ./^n. 

 exotica (Lam.), a South American species, the sinus is generally of 

 the form of an equilateral triangle, the inferior angle being sharp and 

 well defined. In the present species the sinus is still more remark- 

 able, curving in towards the cavity of the beak and terminating with 

 quite an acute angle. The line of the opening of the two specimens 

 is curved and not a plane, as usual with the Naiades ; and the right 

 beak and margin anterior to it, overwrap in a small degree the left 

 beak and valve. In the old specimen this extension of the margin 

 passes the other more than an eighth of an inch — consequently the 

 shell might almost be said to be inequivalve. In its general characters 

 this species most resembles the sinuosa of Lamarck. 



