606 Forty-fifth Report on the State Museum. 



process. External surface covered with lamellar expansions at 

 the concentric growth-lines. 



Tjrpe, Streptis Grayi, Davidson. 



Distribution. Upper Silurian (England and Bohemia). 



Plectambonites, Pander. 1830. 

 (Plate 19, Figs. 1-7.) 



Shells usually small, normally concavo-convex. Surface 

 covered with very fine striae, often alternating in size. Hinge- 

 line making the greatest width of the shell, the extremities often 

 subauriculate. Cardinal area narrow in both valves, sometimes 

 obscurely crenulated on the margins. Pedicle- valve with a 

 moderately broad delthyrium which is partially closed by a con- 

 vex plate, but mostly occupied by the cardinal process of the 

 opposite valve. Apical foramen sometimes retained. Teeth 

 prominent and supported by thickened plates, which are con- 

 tinued in broad outward curves for more than half the length of 

 the valve, returning . and uniting in the unbonal cavity, thus 

 limiting two linguiform muscular scars, inclosing a more or less 

 clearly defined adductor impression. 



In the brachial valve, the dental sockets are deep, and often 

 appear to transect the cardinal area. The cardinal process is 

 simple and erect, but by its coalescence with the short prominent 

 crural plates, the posterior face appears trilobate. The crural 

 plates end abrutly as in Orthothetes; becoming thickened at 

 about the middle of their length, giving origin -to two low 

 ridges or septa, which at first approach each other, and thence 

 continue forward with a slight divergence, thus forming the 

 inner boundaries of two elongate muscular scars, which are less 

 sharply defined on their outer margins. The muscular area is 

 rendered quadripartite by two short transverse or oblique pos- 

 terior furrows. Vascular impressions radial, sometimes digitate 

 Shell-substance fibrous, sparsely punctate. 



Type, Plectambonites planissima, Pander. 



Distribution. Lower — Upper Silurian. 



Christiania, Hall. 1892. 



(Plate 19, Figs. 8-13.) 



Shells usually longitudinally elongated, sometimes semielUp- 



tical in outline; normally concavo-convex. Surface smooth or 



covered with fine radiating lines which are crossed by stronger, 



158 



