Geol.— Vol. II.] ANDERSON— CRETACEOUS DEPOSITS. lO/ 



in every particular that little hesitation is felt in stating the 

 identification. A quotation from Stoliczka's description is 

 applicable to the California species exactly. He says: 

 "The small tubercles on the edge of the back of the young 

 shell, the unequally longer and shorter ribs, the nodular 

 ribs on the back of the body chamber, the irregular evolu- 

 tion of this last chamber, the division of the septa," — all 

 these characters which have been recognized in the Indian 

 examples are clearly seen also in those from California. 



Occurrence. — This species comes from the Horsetown 

 beds of Cottonwood Creek, Shasta County, California. 



53. Acanthoceras compressum, sp. nov. 



Plate IX, Fig. 1S7. 



Shell small, compressed or discoidal; average diameter of adult shell about 

 4.5 cm., greatest thickness 1.5 cm.; height of whorl about twice the width of 

 umbilicus, which is about one-fourth the diameter of the coil; surface marked 

 by flattened and rather flexuous ribs, of which there are about thirty-two in a 

 complete adult whorl; ribs often considerably reduced in strength, especially 

 on the sides of the shell, and ornamented at each extremity with rows of 

 prominent nodes. Along the margin of the umbilicus these tubercles are 

 rather high and narrow, inclining forward, while at the ventral termination of 

 the ribs the prominent linear nodes are often parallel to the median plane 

 in their arrangement. A secondary row of tubercles, less pronounced in 

 appearance, occupies a position inside the marginal row, each one forming a 

 point from which the rib bends rather sharply forward. The ventral surface 

 is flattened or only slightly convex between the marginal nodes, and is gen- 

 erally crossed by faint undulations which are the continuations of the ribs. 

 The median row of nodes sometimes noticed in species of this genus does 

 not appear on any of the specimens of this shell. 



A. compressum is no doubt very closely related to 

 Am. rhotomagensis (var. compressus) Stoliczka, and per- 

 haps might be included in that species with no greater 

 stretch of Stoliczka's definition; but there does not seem 

 to be sufficient reason to include all of his four varieties in 

 a single species, while at the same time other forms are 

 excluded. A. compressum has a near ally in a species 

 from the Lower Chico beds of Southern Oregon, referred 

 to Acanthoceras rhotomagense, which very probably belongs 



