Geol.— Vol. I.] SMITH— COMPARATIVE STRATIGRAPHY. 407 



Pseudomonotis subcircularis Gabb. 



Plate XLIX, Figs. 1-3. 



1864. Monotis subcircularis, Gabb, Pal. Calif. Vol. I, p. 31, pi. vi, figs. 29, 



29a. 

 1886. Pseudomonotis subcircularis, Teller, in Mojsisovics, Arktische Trias- 



faunen, p. 113. 



Form and ornamentation extremely variable; pectinoid, inequilateral, 

 inequivalve, oblique, broadly ovate, with the greatest height towards the rear. 

 Front broadly rounded, rear sloping gently up to the hinge-line. Left valve 

 highly arched, right valve flatter. Hinge-line straight, and a little more than 

 one-third of the total length of the shell. The length of the shell is slightly 

 greater than the height. Anterior ear with byssal notch on right valve, — the 

 character that distinguishes this species from Monotis. Posterior ears alike 

 on both valves. 



Surface ornamented with rather coarse radial ribs about 26 in number, and 

 between most of these there is a finer intercalary rib. The principal ribs begin 

 near the beak, but the intercalaries make their appearance at a height of 

 about 15 mm. The interspaces are considerably wider than the ribs. There 

 are also fine concentric wrinkles or striae of growth over the entire surface. 



In youth the shell is much more elliptical in shape, is longer than high, and 

 the ribs are much fewer, as well as coarser in proportion to the size of the 

 sheU. 



This species is nearly allied to the variety described by 

 Teller^ as Pseudomonotis ochotica Keyserling, var. densis- 

 triata, and indeed appears to be identical with it. The 

 writer is, however, by no means convinced that all the so- 

 called varieties of Pseudomonotis ochotica really belong to 

 one species. Certainly their discrimination is not on a par 

 with that of the ammonites nor of the members of the 

 Pectinidae. 



The writer has also found, in the Pseudomonotis beds of 

 California and Nevada, varieties with fewer and coarser 

 ribs, more like the typical form of P. ochotica, as figured by 

 Teller,^ and it is quite possible that there may be in 

 America, as is said to be the case in Siberia, a transition 

 between these varieties. At present the writer is unable to 

 decide this, and so merely the typical Pseudomonotis subcir- 

 cularis is figured. 



1 Arktische Triasfaunen, p. 116, pi. xvii, figs. 7-15. 

 'Ibid. pi. xvii, figs. i-6. 

 ( 7 ) July 25. 1904- 



