148 



It will be seen that the Lower and Upper Etcheminian faunas are 

 each characterized by a special subgenus of Obolus, one having a peculiar 

 crowded arrangement of the central group of muscles of the dorsal valve, 

 the other having the vascular trunks unusually approximated in the 

 ventral valve. 



Billmgsella 



retroflexa 



described. 



Ornamenta- 

 tion of the 

 surface. 



BILLINGSELLA, Hall & Clark. 



BlLLINGSELLA RETROFLEXA, PI. X., figs 2tt 6. 



This was described as a variety of Clitambonites plana, Pander. But 

 it has been found to belong to a much lower horizon than that species, 

 and to be, by its internal characters, a Billingsella. 



The ventral valve is flat, except toward the umbo, which is elevated, 

 and for a short distance the shell has a convex slope on the back. 

 Interior. The mould of an example from the sandy shale of Young point 

 shows clearly that this is not an Orthisina. The print of the adductors 

 extends forward a little more than a third of the length of the valve, and 

 is surrounded by a low ridge, connecting with the margin of the del- 

 tidium ; a slight emargination at the side of the imprint limits the 

 posterior adductor muscles. The print of the diductors extends far in 

 advance of that of the adductors more than half as far again as these. 

 A lateral septum divides the diductors, and the posterior adductor is 

 bounded by an accessory lateral septum. In front of the print of the 

 adductors is a lanceolate depressed area, bordered by the minor pair of 

 trunks of the vascular system ; the outer pair of trunks turn outward 

 around the print of the diductors, going toward the cardinal angle. The 

 course of the branches of the vascular trunks is straight ; they are 

 numerous, but seldom fork ; their impression is faint beyond the outer 

 resting stage in the growth of the shell. 



The dorsal valve is decidedly tumid in the middle portion, but curves 

 down quickly to the hinge line, where there is a low area. Interior. 

 A mould of the dorsal shows that there was a median septum running 

 from the umbo nearly two-thirds of the length of the shell ; and on each 

 side of the ridge appear three radiating vascular furrows. At the hinge 

 line are the moulds of two short lateral plates, and at the centre of this 

 line, on the mould, a group of three pits due to the crurse and the car- 

 dinal process ; the latter is larger than the other two. 



Sculpture. The material collected last summer has thrown consider- 

 able light on the sculpturing of the valves, which is so different on the two 

 valves as to lead at first to the supposition that there are two species, with 

 difference in the coarseness of the ornamentation one with ribs on the 



