HYRIA. 



seems to us, to be tlie link which connects tlie others 

 with Lamarck's Conques Jluviatiles. 



The Hyrise are equivalve, inequilateral, transverse, 

 obliquely subtriangular shells, with two wings; in their 

 general form, they consequently very nearly resemble the 

 Aviculce, but they are generally more gibbose, and their 

 two lateral muscular impressions, of which the posterior 

 one is compound, will also distinguish them from Avicula, 

 without taking into consideration the characters of the 

 hinges. The base according to Lamarck, or the hinge 

 line according to Swainson is elongated, and generally 

 very straight. Hinge with two elongated teeth in each 

 valve, the posterior of which is divided irregularly into 

 several parts, and of these the internal are the smaller, 

 and the external larger and lamellar, and forming two 

 lamellar portions in one valve ; the anterior or lateral 

 tooth much more lengthened, lamellar, and bipartite in 

 the other valve. The teeth are rough and irregularly 

 striated. Muscular impression of the mantle entire. 

 Ligament external, linear. A dark olivaceous coloured 

 epidermis covers the outside wherever it has not been 

 eroded, which has generally been the case with the more 

 prominent parts of the shells. 



We are at present acquainted with only four species 

 of Hyria; H. avicularis, iam. (Mya syrmatophora, Z/mw.) 

 H, corrugata. Lam. H. elongata, Swains, and another in 

 our own collection ; all these are recent from the rivers 

 of warm climates. We have never seen any fossil species. 



