CARBONIFEROUS AGE. PERMIAN PERIOD. 57 



Bakevellia parva. 



(Plate II, Fig. 12, a, 6.) 

 Bahevellia parva, Meek & Hatden, Trans. Albany Inst. IV, March. 2d, 1858. 



Shell very small, obliquely subovate, oblong, or subrhombic in outline ; valves gibbous along the oblique umbonal 

 slopes. Antero-ventral margin sloping very obliquely backwards and downwards ; rather distinctly sinuous under 

 the beaks. Postero-basal extremity rounded ; anal edge sinuous above ; anterior extremity somewhat lobed, sub- 

 angular or very narrowly rounded. Hinge line straight, nearly or quite equalling the greatest length of the shell, and 

 ranging at an angle of about 35° to the umbonal prominences. Beaks rather small, rising a little above the hinge, 

 incurved, and located about half way between the middle and the anterior extremity. Postero-dorsal region com- 

 pressed, or more or less alate, and terminating at a distinct angle at the extremity of the hinge. Surface with 

 concentric strise. Anterior teeth of hinge, one or two to each valve, linear and declining a little in front ; posterior 

 teeth, one or two, long, linear and ranging parallel to the hinge margin. 



Length, 0.20 inch ; height, 0.10 inch ; thickness or convexity of the valves, about 0.08 inch. 



We can scarcely regard it as a clearly established fact that this little shell 

 possesses all the characters of Prof. King's genus Bahevellia, since its cardinal area 

 and muscular and paUial impressions have not yet been seen. It agrees, however, 

 so exactly in form and general appearance, as vs^ell as in the nature and arrange- 

 ment of its hinge teeth vv^ith that genus, and differs so materially in the latter 

 character and the absence of a byssal sinus from Pteria, that we do not feel war- 

 ranted in removing it from the genus in which we first provisionally placed it. 



It agrees very nearly in form with some varieties of Bahevellia antiqua, Munster 

 (sp.), but is uniformly much smaller — never being more than one-fourth the average 

 size of that shell ; while its cardinal area must be much narrower, judging from 

 the close proximity of the beaks ; the posterior extremity of its hinge is also much 

 more angular. 



Locality atid position. — Near the mouth of Smoky HiU fork of Kansas Eiver; 

 on Cotton-Wood Creek, and at numerous other localities in northeastern Kansas ; 

 in beds of yellowish magnesian limestone of Permian Age. (Type 3959.) 



Family TEIGONIIDiE. 



Shell equivalve, generally inequilateral, closed, varying greatly in 

 form and ornamentation, according to the several genera and smaller 

 groups, nacreous within. Ligament external ; hinge composed of a few 

 diverging, usually large, interlocking teeth. 



Animal with two recumbent gills on each side ; palpi simple ; mantle 

 open ; foot long, lanceolate, bent, and formed for leaping. 



This family includes the following genera : Schizodus, Myoplioria, Trigonia, and 

 Verticordla ? The first two of these genera are entirely extinct, while the Trigonias, 

 which were most abundant during the Jurassic and Cretaceous epochs, are still 

 represented by a few living species. Of the genus Verticordia, only one or two 

 Tertiary species, and one living species are known. 



In tracing the different groups of this family, from its first appearance in the 

 Palaeozoic rocks, as plain shells (ScJdzodus) with smooth Aveak hinge teeth, it is 



8 May, 1864. 



