ORDER I PRIONODESMACEA 367 
obsolete in the adult, though present in the young ; generally byssiferous ; hinge Schizodont 
or edentulous. The young sometimes showing a distinct nepionic stage. Marine. 
Family 15. Pterineidae. Dall. 
Shell Pteriiform, bialate, dimyarian, the anterior adductor smaller ; inequivalve, very 
inequilateral ; dentition obscure; ligament amphidetic, external, multivincular (2); the byssus 
passing through @ notch in the smaller valve. Ordovician to Devonian. 
In Pterinea and its allies we have the first indications of divergence of what ultimately 
became Taxodont and Schizodont dentition. From this assemblage, as indicated by Jackson, 
a large proportion of the Prionodesmacea have diverged in various directions. It 1s probable 
that from this source the filibranchiate Taxodonts have sprung, rather than directly from the 
foliobranchs. 
Rhombopteria, Jackson (Fig. 617). Posterior wing separated from the body of the 
valve by a shallow sinus ; anterior wing short ; teeth obscure, the posterior elongated. 
Silurian. ' 
Pterinea, Goldf. (Fig. 618). Left valve convex, right valve flat ; hinge plate long, 
broad, auriculate before and behind ; area amphidetic, grooved ; ligament parivin- 
A B 






Fic. 617. 
Rhonbopteria mira, Fie. 618. 
Barr. sp. Silurian (B); A, Pterinea laevis, Goldf. Devonian; Niederlahnstein, Nassau. Interior of left 
Prague(after Jackson). valve, 1/;. B, P. lineata, Goldf. Same locality ; external view. 
cular (?); anterior teeth obscure, transverse ; the posterior elongate, nearly parallel to 
the cardinal margin, depressed behind. Posterior adductor scar large, the anterior 
small but strong, inserted below the anterior wing. Silurian to Carboniferous ; 
particularly abundant in the Devonian of Europe and America. 
Actinodesma, Sandb. (Glyptodesma, Ectenodesma, Hall ; Dolichopteron, Maurer). Like 
Pterinea, but with the wings elongated and pointed. Devonian. 
Leptodesma, Hall; Kochia, Frech (Onychia, Sandb.; Loxopteria, Frech). Devonian. 
Family 16, Lunulicardiidae. Fischer. 
Usually equivalve, triangular shells with terminal beaks, from which a sharp ridge 
runs toward the lower margin, bounding a flattened area. Hinge margin straight, long. 
Internal characters unknown. Silurian and Devonian. 
Lunulicardium, Miinst. Anterior side with a byssal sinus. Silurian and 
Devonian. L. semistriatum, Miinst. 
Patrocardium, Fischer (Hemicardium, Barr, non Cuvier). Without byssal sinus. 
Silurian. 
Additional genera: Amita (Spanila, Tetinka), Mila, Tenka, Babinka (Matereula), 
Barrande. Silurian, 
Family 17. Ambonychiidae. Miller. 
Shell Mytiliform, with no anterior wing, the anterior adductor obsolete; equivalve, 
very inequilateral ; dentition obsolete or Schizodont ; ligament external, multivincular (?) ; 
