Geol.— Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 403 



Genus Halobia Bronn. 



Type, Halobia salinarii^n Bronn. 



Generic characters (as defined by Mojsisovics, Ueber die Triadische 

 Pelecypoden-Gattungen Daonella und Halobia, Abhandl. k. k. geol. Reich- 

 sanstalt, Wien, Bd. VII, Heft 2, 1874, p. 23): Bivalve, inequilateral, beak 

 conical, moderately high, situated anterior to the middle of the hinge area, 

 which is long, straight and taking up the full length of the shell. Valve 

 wider than high. Anterior ear, on both valves, separated from the rest of 

 the shell by a depression, and distinguished by the change in ornamenta- 

 tion. No byssal notch. Radial ribs over the whole surface, and usually 

 concentric wrinkles. 



Halobia s. str. begins in the lower Ladinic stage, top of 

 the Middle Trias, but becomes common first in the Karnic 

 stage of the Upper Trias, to which horizon it is chiefly 

 confined. It is found in the Upper Trias of the Alps, In- 

 dia, Spitzbergen, the Indian Ocean, and California. It is 

 rare in the Noric stage. Halobia developed out of Daonel- 

 /«, with which it is connected by transitional forms. 



Halobia superba Mojsisovics. 



Plate XLVIII, Figs. 1-2. 



1874. Halobia superba, Mojsisovics, Ueber die Triadischen Pelecypoden- 

 Gattungen Daonella und Halobia, p. 30, pi. iv, figs. 9-10. 



Beaks rather high and conical. Form highly arched, though usually 

 pressed flat, wider than high. Surface ornamented with fine radial ribs, 

 numerous and close-set, with interspaces narrower than the ribs. The ribs 

 bifurcate, and are not grouped in bundles, strongest on the arch of the 

 valve, weakest on the front and rear. At the height of 12-15 vam. the ribs 

 bend suddenly forward at a strong concentric wrinkle, and then bend as 

 suddenly back again. This is characteristic on all specimens, and marks the 

 time of attachment of the shell. Concentric wrinkles faint but distinct, 

 strongest on the young shells. Anterior ear distinct, cut off from the shell 

 by a furrow and change in ornamentation. 



The size at which the angle in the ribs forms is usually 

 smaller than that given by Mojsisovics, twelve instead of 

 fifteen millimeters, but this character is not sufficient for 

 specific discrimination. Several Californian specimens 

 showed height of fifteen millimeters at the angle. 



The beak has only concentric wrinkles and no ribs; this 

 stage probably corresponds to the genus Posidonomya, and 

 lasts up to a diameter of two and five-tenths millimeters. 



