bal) VCs La 
Puate I. 
Genus HYRIA, Lamarck. 
Testé bivalvis, bimusculosd, fluviatilis, intis nacrea 
eats epidermide corned indutd, trigond, postice 
angulata, plusminusve auritd ; cardo dentibus pos- 
ticis elongatis, lanvinatis oblique serratis duobus 
et uno, centralibus in utraque valvd duobus parvis, 
anticis brevibus, laceratis wno et duobus. 
Shell bivalve, bimuscular, fluviatile, pearly within, 
covered outside with a horny epidermis, trigonal, 
angular posteriorly, more or less aurited. Hinge 
with two and one posterior, elongated, laminated 
obliquely serrated, two central, small in each valve, 
and two and one short, lacerated teeth. 
The characters of the Genus Hyria as distinguished 
from the numerous other forms of Unionide, are more 
appreciable to the eye than definable in description. 
To pick out an Hyria from a large group of Nayades, it 
is necessary to take in not one defined character, but an 
assemblage of characters presenting a distinctive appear- 
ance in their totality. Thus the Hyriz have auricles ; 
so have some Unios. They have a posterior angle and 
are trigonal in form; so are many of the larger groups. 
And the long laminar striated posterior teeth, with the 
short lacerated anterior ones, although very characte- 
ristic, are by no means exclusively so. Yet almost 
every conchologist distinguishes an Hyria when he sees 
one. The Hyria form of the Freshwater Mussel appears 
to be peculiar, with one exception, to the Amazon and 
other rivers in South America. T'wo species only, the 
H. avicularis and H. corrugata are enumerated by 
Lamarck. The list is swelled by Hupé in Castelnau’s 
Expedition in South America to seven, and the present 
monograph brings it up to thirteen, including H. con- 
torta, which is the only species not inhabiting South 
America, being from Shanghai. 

Species 1. (Mus. Sowerby.) 
Hyrra corruaata. Myr. testa solidd, trigond, tumidd, 
intis salmoned, extits prope umbones rugis magnis 
obliquis undulatis divaricatis ornatd, versus 
margines leviusculd ; latere postico lato, elevatim 
angulato, ared postangulari elevata, pland,  tri- 
angulari, magnd ; latere antico angusto, brevi ; ald 
parva, acutd. 

THE corruGATED Hyrta. Shell _ solid, 
tumid, salmon-coloured within, ornamented out- 
trigonal, 
side near the umboes with large oblique undulat- 
ing diverging ridges, smoother towards the mar- 
gins; posterior side wide, with an elevated angle; 
postangular area elevated, flat, triangular, large ; 
anterior side narrow, short ; wing small, acute. 
Lamarck. Anim. S. vert. Species 2. 
Triplodon rugosus.  Spix. 
Hab. British Guayana and Brazil. 
This comparatively short, tumid, angular form 
appears to be the one generally accepted for Lamarck’s 
species, while several other forms to be enumerated 
have either been considered varieties of this, or as worthy 
of new names, which they have not hitherto received. 

Species 2. (Mus. Brit.) 
Hyr. testa lanceolata, transversa, 
valde prope umbones 
noduliferd, anticé alatd, medio arcuatim depressa ; 
valvulis crassis ; wmbonibus parvis ; latere postico 
elongato, prope marginem angulato, valdé arcuato, 
Hyria cCONnToRTA. 
tortuosa, inequilaterali, 
termino subquadrato ; ligamento elongato, magno ; 
latere antico brevissimo, arcuato, acuminatim pro- 
ducto. 
TWISTED Hyrta. 
tortuous, very unequilateral, nodulose near the 
umboes, alated in front, arched and depressed in 
the middle, valves thick, umboes small; posterior 
side elongated, angular near the margin, much 
arched, end subquadrate; ligament elongated, 
large; anterior side very short, arched, acumi- 
THE Shell lanceolate, transverse, 
natedly produced. 
Triquetra contorta, Obs. Unio., vol. vi. p. 39. 
Shanghai. 
Lea. 
Hab. 
The appearance and characters of this shell certainly 
tend to associate it with the present genus, notwith- 
standing its singularly curved and lanceolate form. 
The former character, when only a young undeveloped 
specimen was possessed, was suspected to be only acci- 
dental ; but adult specimens having been procured from 
China with exactly the same peculiarity, that question is 
setatrest, and it is established that, as in the cases of Arca 
tortuosa and Tellina elegans, the curved form is normal. 
April, 1869. 
