Prof. H. A. Nicholson—Organisms in Paleozoic Limestones. 15 
with the massive and solid bones of the skeleton, and, especially, 
the enormous vertical extent of the compressed tail, indicate an 
aquatic life. This opinion was expressed by the writer in describ- 
ing the first specimen found, and the discoveries since made have 
done much to confirm it. That these reptiles moved freely on land, 
also, is quite probable. Other genera of the group may have lived 
mainly upon the land. 
The large number of specimens of the Stegosauria now known 
from the American Jurassic, and the fine preservation of some of the 
remains, enable us to form a more accurate estimate of the relations 
of the group to the other Dinosaurs, than has hitherto been possible. 
The presence of a predentary bone, and the well-developed post- 
pubis, are important characters that point to the Ornithopoda as near 
allies, with a common ancestry. These positive characters are sup- 
plemented by some points in the structure of the skull, and the form 
of the teeth. 
There are, however, a large number of characters in which the 
Stegosauria differ from the Ornithopoda, and among these are the 
following :— 
(1) All the bones of the skeleton are solid. 
(2) The vertebre are all biconcave. 
(3) All the known forms have a strong dermal armour. 
(4) The second row of carpals and tarsals are unossified. 
(5) The astragalus is codssified with the tibia. 
(6) The spinal cord was greatly enlarged in the sacral region. 
The relations of these two groups to each other and to the rest of 
the known Dinosauria will be fully discussed by the writer in his 
monograph on the Stegosauria. 
New Haven, Conn., October, 1887. 
IIJ.—On cEertTAIN ANOMALOUS ORGANISMS WHICH ARE CONCERNED 
IN THE FORMATION OF SOME OF THE PaL#ozo1c LIMESTONES. 
By H. Atteyne Nicuotson, M.D., D.Sc., F.G.S., 
Regius Professor of Natural History in the University of Aberdeen. 
ee many of the Paleozoic limestones are more or less exten- 
sively composed of the skeletons of various Invertebrate 
animals, sometimes in a perfect condition, sometimes more or less 
largely fragmentary, has long been known. In certain instances 
a microscopic investigation of these ancient calcareous sediments may 
fail to demonstrate the presence of organic remains, or may reveal 
but few of these. Thus there occur beds of lithographic limestone 
in the Paleeozoic series which would seem to be simply of the nature 
of very finely levigated calcareous mud, the component grains of 
which were, however, doubtless derived, in the first instance, from 
the calcareous skeletons of animals. Again, it commonly happens, 
even in examples where the rock may to all appearance be little 
altered, that a limestone may be found on examination by means of 
thin sections to have undergone secondary crystallization, with the 
result of a more or less complete obliteration of the organic remains 
