138 PALEONTOLOGY OF KENTUCKY. 



from which it differs, however, by its smaller size and smaller number of plications ; our shell has only ten 

 ribs on each valve, while that species has, according to Prof. Hall, from twelve to sixteen ; then, again, 

 our specimen has only one rib, which bifurcates below in the depressions, while Tr. globosa has always 

 two, often three, in said depressions. 



Genus Zygospira. Han. 



Zygospira, Hall. 15th Regent's Keport, 1862. 

 Etymology : zygos, a yoke ; spira, a spire. 



This genus was established by Prof. Hall to receive such forms as Zygospira 

 modesta, Z. cincinnatiensis and Z. headi. 



Z. modesta is made the type of the genus, which has great similarity to Z. 

 kentuckiensis, and which latter, therefore, presents the main characteristics of 

 the genus. The original description of this genus, by Prof. Hall, is cited from 

 his 15th Regent's Report. It is to be regretted that Prof. Hall's valuable 

 writings are scattered through such a great number of scientific periodicals 

 and State reports. 



Zygospira kentuckiensis. . SP. 



Plate XXXIV., figures 21 to 25. 



Shell rather large, as compared with other species of Zygospira ; sub-circular 

 or sub -elliptical ; from moderately convex to gibbous ; cardinal lines sloping 

 towards the front, forming a very obtuse angle at the beaks ; lateral margins 

 regularly curved; front broadly rounded, with a straight or sinuate central 

 part. Ventral valve larger than the dorsal ; central part elevated, and forming 

 a mesial fold, extending from beak to front, not defined at its margins, and of 

 moderate elevation. The middle line of this fold is marked by a somewhat 

 broader and deeper furrow, towards which both sides of the fold slightly in- 

 cline, thus forming a small mesial sinus on the top or summit of the fold ; this 

 central depression on the fold extends also from the beak to the front. The 

 lateral slopes of the mesial fold are more or less abrupt, and the surface be- 

 tween fold and sides becomes flat and even somewhat concave ; the umbo is 

 prominent and the beak pointed, and, in most shells, closely incurving upon 

 the beak of the opposite valve, concealing entirely the hinge-area and its fora- 

 men. 



In a few specimens in my collection the beaks of both valves are sufficiently 

 apart to show a small ventral hinge-area, with a small triangular fissure. 



The dorsal valve is moderately convex in its lateral parts ; its central portion 

 is depressed, forming a mesial sinus, which extends from the apex to the 

 front, and increases gradually in depth and width towards the base ; the umbo 

 is inflated, and the beak minute and incurved. The surface is covered by 



