276 . AVICULlDiE. 



obtusely winged, beaks prominent, turned anteriorly, surface 

 smooth, witli growtli-liues ; tlie liinge shows a ridge directed 

 posteriorly, on each side of which 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 usuall}^ bifid ; pallial impressions 

 unknown. 



MYALiNADONTA, CEhlert, 1882. Teeth differently disposed, the 

 form is flattened and the cardinal margin is perpendicular to the 

 axis of the shell. 



Mytilaroa, Hall, 1869. 



Distr. — 13 sp. Fossil; Chemung Group, JST. Y. ; Germany. 

 M. Chemungensis, Conr. (cxxx, 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 border nearlj' straight, somewhat sinuate on the margin, 

 apparently for the accommodation of byssal threads ; posterior 

 border usuall}^ subparallel with the anterior, wdiile the basal 

 margin is generally abruptly rounded like that of Mytilus ; 

 muscular markings very like Mytilus ; surface of valves often 

 obscurely radiately striate. 



Plethomytilus, Hall, 1883. 



Distr. — Palaeozoic ; New York. Inoceramus mytilimerus, 

 Conrad. 



Shell mytiloid, gibbous, with a finel3f striated ligamental area ; 

 hinge edentulous so far as observed. DiflJers from Mytilarca in 

 its erect form, shorter transverse hinge-line, absence of cardinal 

 and lateral teeth, and non-truncate anterior side. 



LiMOPTERA, Hall, 1869. 



Distr. — 5 sp. Fossil. Upper Helderberg and Hamilton 

 Groups ; New York, Kentucky. L. 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, verj^ small and deep ; 

 posterior one large, a little behind the middle 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). 



