Report of the State Geologist. 



591 



Bemipbonttbs, Pander, L830. (Plate 8, Figs. ll. i.v Shells 

 as in CuTAMBONrraa but with shorter hinge-line and subequally 

 convex valves. Type. //. imrdda, Pander; Lower Silnrian). 



Orthisina D'Orbigny, 1847. (Plate 8, Fig 8 LO L3.) Shells as 

 in Clitambonites bu1 with the cardinal area and ohilidium of the 

 brachial valve extravagantly developed. Type, 0. anomala, 

 Schlotheim (sp.) Lower Silurian). 



Subgenus Polytcechia, Ball. L892. 

 (Plate 8, Pigs, lfr-20.) 

 Shell small, subtrihedral in contour. Hinge-line straight, 



aboul equaling the diameter of the shell. Pedicle-valve with a 



high,nearly vertical cardinal area marked with oblique striations 



parallel to the lateral margins. Delthyrium coveredbv a narrow, 



convex plate; the presence of a foramen 



not determined. On the interior the 



dental lamellae are widely separated, and 



descend alon^ the umbonal cavity for a 



short distance vertically, thence bending 



sharply inward and meeting at a low 



angle in the median line; thus forming Poiytceckta apicaii*. 



with thedeltidium. a conspicuous subrostral F ' ° ( ^SSTLTS 



vault. This is supported by a stout median a P<*- 

 septum, and two smaller lateral septa, which meet it at the lines 

 of angulation; the former of these extends for the entire length 

 of the plate, while the latter is free from the accessory septa 

 near its anterior edge. The umbonal cavity of the valve is thus 

 divided into five chambers, and in the lateral chambers there is 

 still another septum, lower than the rest and not extending to 

 the spondylium. The brachial valve is shallow and depressed- 

 convex, with a narrow cardinal area. The delthyrium is very 

 broad with a partially developed covering, the dental sockets are 

 widely separated, the crural plates narrow and nearly parallel to 

 to the hinge-line. The cardinal process is simple, linear and quite 

 prominent, and at its union with the crural plates is a subtri- 

 angular thickening which is supported by a low median septum. 

 Surface covered with tine, elevated, radiating striae, without 

 evidence of median fold and sinus. 



T yP e > Hemipronites apicalis, Whitfield. 



(Lower Silurian.) 



143 



