Geol.— Vol. II.] ANDERSON— CRETACEOUS DEPOSITS. 103 



arise from within the umbilicus and pass outward to the ventral side, while 

 some of them arise from tubercles upon the umbilical shoulders. About six 

 constrictions cross the outer whorl transversely, bordered by ridge-like ribs 

 behind. As the shell increases in diameter it becomes rapidly more dis- 

 cpidal, narrowing toward the ventral edge. The ribs curve more strongly 

 forward in approaching the siphonal margin, the tubercles upon the umbil- 

 ical shoulder become obsolete or indistinct, and the height of the whorl 

 increases considerably. The more inflated form of the young shell of this 

 species is probably represented by Gabb's species, Ammonites fraternus} 



Occurrence. — The species belongs to the upper portion 

 of the Chico beds, having a wide distribution in this 

 horizon. 



48. Pachydiscus merriami, sp. no v. 

 Plate VI, Figs. 135-138, 

 cf. Ammonites suciaensis Gabb (not Meek), Pal. Cal., Vol. I, PI. XXVII. 



Shell robust, but little compressed, rounded on the abdomen, and with 

 small umbilicus; walls of umbilicus abrupt within, rounded upon the 

 shoulders, deep and somewhat funnel form; width of umbilicus less than 

 one-fifth the whole diameter of the shell; somewhat flattened upon the sides, 

 rounded broadly over the ventral surface, and very thick; surface marked 

 with about eight transverse, shallow grooves, which are seen only upon the 

 casts, while upon the shell itself there are as many rounded ridges that 

 border these grooves in front; ridges more prominent upon the ventral 

 surface and almost disappearing upon the sides; lines of growth distinct 

 between the ridges. 



The measurements of the largest specimen found are: diameter, 9.7 cm.; 

 greatest thickness, 4.7 cm.; width of umbilicus, 1.7 cm.; depth of involution, 

 1.7 cm.; height of last whorl from umbilicus, 4.8 cm. The suture consists of 

 two principal and three smaller auxiliary lobes, diminishing rapidly in size. 

 Both lobes and saddles are much divided, the saddles consisting in their 

 final divisions of broadly denticulated digitations that are somewhat spatulate 

 in form. The terminal branches of the lobes are narrowly acuminate. 

 Shells of this species are nearly spherical at a diameter of i cm., with a 

 reniform section; the depth of whorl becoming proportionately greater with 

 age. In crossing the sides of the whorl the grooves curve at first gently 

 backward and then forward, and approach the median plane obliquely. 



This species is probably the one which Gabb found 

 upon the Cottonwood, in Shasta County, California, and 

 referred to as ^. sticia'cnsis Meek. The figure in the Pale- 

 ontology of California,^ however, was drawn from a speci- 



1 Pal. Cal., Vol. 11, p. 137, PI. XXIII. 



2 Vol. I, PI. XXVII. 



