UNiONiD-a;. 237 



Trigonodus, Saudberger, 1864, 

 Distr. — T. Sandbergeri, Albert! (cxxi, 29, 30 \ Triassic. 

 Shell like Cardinia, hinge also nearly the same, except that 



the posterior lateral tooth is very strong, single in the right and 



double in the left valve ; anterior lateral tooth short and small ; 



cardinal teeth distinct ; muscular impressions elongated and 



attenuated above. Closely allied to Cardinia. 



Cardinia, Agassiz, 1841. 



Etyvi. — Cardo-inis^ a hinge. 



Syn. — Thalassides, Berger, 1833 (no description). Sinemuria, 

 Christol. Pachyodon, Stutch. Ginorga and Dihora, Gray. 

 Storthodon, Brown. 



Distr. — Fossil, 71 sp. Silurian — Inferior Oolite; Europe, 

 along with marine shells. C. Listeri, Sowb. (cxxi, 31). 



Shell trigonal or ovately elongated, compressed, inequilateral, 

 with pointed, not very prominent beaks, these being close 

 together ; hinge with one cardinal tooth in the right valve and 

 two small ones in the left, and one remote lateral tooth on either, 

 side in each valve ; muscular impressions deep, rather small, and 

 situated next to the lateral teeth; ligament of moderate strength, 

 but long. 



This genus is sparingly (and rather doubtfully) repi-esented in 

 Silurian rocks ; its maximum of development falls in the Lias, 

 and it disappears with the close of the Jurassic period. 



( Naiades.) 

 Family UNIONID^. 



Shell usually regular, equivalve, closed ; structure nacreous, 

 with a very thin prismatic-cellular layer beneath the epidermis ; 

 epidermis thick and dark ; ligament external, large and prom- 

 inent ; margins even ; anterior hinge-teeth thick and striated, 

 posterior laminar, sometimes wanting ; adductor scars deeply 

 impressed ; pedal scars three, distinct, two behind the anterior 

 adductor, one in front of the posterior. 



Animal with the mantle-margins united between the siphonal 

 orifices, and, rarely, in front of the branchial opening ; anal 

 orifice plain, branchial fringed ; foot very large, tongue-shaped, 

 compressed, byssiferous in the fry, and sometimes in the adult; 

 gills elongated, subequal, united posteriorly to each other and 

 to the mantle, but not to the body ; palpi moderate, laterally 

 attached, striated inside ; lips plain. Sexes distinct. 



The river-mussels are found in the ponds and streams of all 

 parts of the world. In the old world the species are comparatively 



