276 AVICULIDA. 
obtusely winged, beaks prominent, turned anteriorly, surface 
smooth, with orowth- lines; the hinge shows a ridge directed 
posteriorly, on each side of W hich is the ligamental area, covered 
with parallel grooves; these grooves are subparallel with the two 
margins of the shell, but whilst they are regular and straight on 
the ‘posterior side, they are curved anteriorly ; cardinal teeth 
three, the anterior one usually bifid; pallial impressions 
unknown. 
MYALINADONTA, Cihlert, 1882. Teeth differently disposed, the 
form is flattened and the cardinal margin is perpendicular to the 
axis of the shell. 
Myriptarca, Hall, 1869. 
Distr.—13 sp. Fossil; Chemung Group, N. Y.; Germany. 
M. Chemungensis, Conr. (¢xxx, 51). 
Shell equivalve, mytiliform; beaks terminal and hinge-line 
straight; anterior end of hinge-plate with one to three rather 
strong oblique teeth in each valve, with corresponding cavities ; 
posterior end with from two to four often obscure, parallel con- 
tiguous tooth-like ridges, the marginal one parallel with the 
posterior border of the valve, the others shorter and divergent ; 
anterior Lorder nearly straight, somewhat sinuate on the margin, 
apparently for the accommodation of byssal threads; posterior 
border usually subparallel with the anterior, while the basal 
margin is generally abruptly rounded like that of Mytilus ; 
muscular markings very like Mytilus; surface of valves often 
obscurely radiately striate. 
PviernomytTitus, Hall, 1883. 
Distr.—Paleozoic; New York. IJnoceramus mytilimerus, 
Conrad. 
Shell mytiloid, gibbous, with a finely striated ligamental area ; 
hinge edentulous so far as observed. Differs from Mytilarea i in 
its erect form, shorter transverse hinge-line, absence of cardinal 
and lateral teeth, and non-truncate anterior side. 
Limoprera, Hall, 1869. 
Distr.—5 sp. Fossil. Upper Helderberg and Hamilton 
Groups; New York, Kentucky. JL. pauperata, Hall. 
Shell large, inequilateral, inequivalve, the right valve smaller ; 
ligamental area large, longitudinally striate, and extending to 
the extremity of the wings; anterior margins sinuated, forming 
an elongated byssal opening; hinge edentulous? anterior 
muscular impression within the umbones, very small and deep; 
posterior one large, a little behind the middie of the shell, and 
nearer the hinge than the base; pallial line simple, formed of a 
series of small, deep pits (or, as seen in the casts, of a series of 
small nodes). 
