1893.] Geology and Paleontology. 657 
externally, although the backbone runs downward to terminate in its 
lower lobe. In this respect the fin has the same general aspect as in 
the Sharks, except that in the latter the backbone runs into the upper 
lobe. It shows, indeed, as Dr. Fraas remarks, how closely analagous is 
the form of Ichthyosaurs to fishes.” (Natural Science Vol. I, 1892.) 
New Fossil Fishes from the Upper Lias.—Continuing his 
studies of the fossil fish from the Upper Lias of Cement of Vassy of the 
Youne, M. Sauvage describes and figures three new species; Leptolepis 
afinis Sauvage, L. antissiodorensis Sauvage, and Pholidophorus 
gaudryi Sauvage. The first resembles L. constrictus Egerton, but 
differs from that species by having the preopercular strongly striated, a 
longer body, more numerous vertebre, and the ventral fins further 
back. Polidophorus gaudryi is very close to P. bechei Ag. and P. ony- 
chius Àg., it has, however, a longer head, and the scales not so elevated : 
and less numerous than either of these species. It may be identical 
with P. dorsalis Ag., but the description of that species is too meager to 
allow full comparison. (Bull. de la Soc. d'Hist. Nat. d'Autun, 1892.) 
Affinities of Ichthyornis.—Dr. R. W. Shufeldt has published a 
tabular comparison of the anatomical points of Ichthyornis dispar, 
Rhynchops nigra and Sterna macrura to show that while in some minor 
characters Ichthyornis seems to come nearer the Gulls than it does the 
Terns, on the whole it possesses more in common with our now existing 
Rhyncopide than with the Sternine. This is most apparent in the 
cranium ; in the large skull compared with the rest of the skeleton; 
and in certain characters in the vertebral chain and pelvis. (Journ. 
Anat. and Physiol. Vol. XXVII.) 
Cretaceous Formations of Mexico.—Mr. R. T. Hill has 
recently shown that the Mountain Limestone so widely distributed 
throughout Mexico is the southern continuation of the Comanche Series 
of Texas, and is therefore of Lower Cretaceous age instead of Upper 
Cretaceous and Paleozoic, as has been asserted. The author has 
recognized the three prominent stratigraphic divisions of the Comanche 
Series in Mexico as in Texas, and is confident that when the whole 
region is studied more detailed resemblances of horizons will be 
observed. 
The true Upper Cretaceous is characterized by shallow deposits of 
ferruginous limestones, clays, sand and lignite. The beds occur in the 
northeastern border States of Chihuahua, Coahuila and Tamaulipas, and 
