424 A. HYATT — TRIAS AND JURA IN THE WESTERN STATES. 



young of P. skidegatensis, Whiteaves, as figured by him * The regularity 

 of the bifurcated costations of these fragments is different from those of 

 filiplex, but these may have been interrupted by more or less single costse 

 on other parts of the whorl, and in that case they would be identical. 

 One fragment showing a portion of the inner whorls was found by Dr 

 Curtice six miles from Copperopolis, on the road to Sonora and grade to 

 Angels creek. If, as is supposed, this be identical with the species here 

 described, the first four whorls or thereabouts have a close resemblance 

 to the young of P. muhlbachi, but the costations of the next or fifth whorl 

 become more widely distributed and similar to those of the larger speci- 

 mens described above. The increase of the dorso-ventral diameters is 

 not so rapid as in muhlbachi and the sides of the whorls are not so 

 broad, the whole shell being more discoidal. 



Perisphinctes coJfaxi. 



Ammonites colfaxi, Gabb. Am. Jour. Conch., vol. v, pt. 1, 1869-70, p. 7, 



pi. 4, fig. 2. 

 Loc, railroad cut one mile west of Colfax. 



This important specimen, which has been lately brought to light in 

 the collections donated to the Museum of Comparative Zoology by Pro- 

 fessor J. D. Whitney, turns out to be entirely distinct from what it has 

 been supposed to be by most authors. Gabb's figure in the American 

 Journal of Conchology does not give the great prominence and true 

 facies of the ribs, but does show the purely perisphinctean characteristics 

 of these parts. The specimen has from ten to twelve whorls and is of the 

 size of Gabb's figure. The living chamber is over one-half of a volution 

 in length. The whorls increased very slowly by growth, the whole shell 

 being discoidal ; the inclusion is slight, covering only the abdomen. The 

 form of the whorl in section had rounded, some^vhat compressed sides 

 and a rounded, broad abdomen. The ribs are more regular than in 

 Gabb's figure, their irregularity in curvature being largely due to com- 

 pression. The rule in the neanic stage is for each alternate rib to be 

 bifurcated, the parts between the bifurcations being elevated and the 

 bifurcation on the exterior of the shell was probably coarsely tubercu- 

 lated or nodose. This gives the bifurcated ribs a very coarse, massive 

 aspect. The alternating single costse are more acute and reach entirely 

 across the sides and abdomen. There are sometimes two or three single 

 costse between the bifurcated ones, and these irregularities increase on the 

 last whorl. The bifurcations are about the middle of the side, but may 



*Mesozoic Fossils, vol. ii, pt. 1, pi. 9, fig. 1. 



