238 
JOURNAL OF MAMMALOGY 
NOTES ON TERTIARY SIRENIANS OF THE GENUS 
DESMOSTYLUS 
By Harold Hannibal 
{Plates 11-12] 
During the past four or five years I have collected numerous teeth 
of extinct sirenians of the genus Desmostylus from the Miocene and 
transitional Oligocene-Miocene beds near San Jose, California. 
The teeth of Desmostylus are of a unique type. When the tooth 
emerges from the gum it is made up of cone-shaped columns of enamel 
filled with dentine. As it wears it becomes a group of closely appressed 
enamel rings surrounding pits from which the dentine has been eroded. 
A number of teeth together with bones and fragments of tusks were 
foimd in company with marine invertebrates in shell-limestone inter- 
calated with the lower or buff sandstone member of the San Pablo 
formation on the San Jose Quadrangle between Monument Peak and 
the saddle where the road to Calaveras Valley crosses the first ridge 
of the Diablo Range. At a horizon only a few hundred feet higher a 
tooth was found in shell-limestone about a half a mile south of the 
saddle where the road to Mount Hamilton crosses the first ridge. A 
fragment of a Desmostylus tooth was also found in a mixture of lime- 
stone and rhyolite tuff interbedded with Monterey shale on the New 
Almaden Quadrangle northeast of the Guadalupe quicksilver mines. 
It was associated with Pecten andersoni Arnold and therefore comes 
from beds considerably older. 
Desmostylus hesperus Marsh^ on which the genus was founded was 
described from fragments of teeth and vertebrae collected by L. G. 
Yates in the Miocene of either Alameda or Contra Costa Counties, 
California. While the exact locality where the type was collected 
will probably never be known, the figure compares closely with a 
molar fragment found at the Monument Peak locality which lies only 
a few miles to the south. At several points in the San Pablo beds of 
Alameda and Contra Costa Counties I have seen fragments of verte- 
brae that I took to be sirenian remains and it is probable that Marshes 
material came from this horizon. 
The teeth from Monument Peak come from different parts of the 
mouths of many individuals. The most common teeth are first molar 
1 Marsh, O. C.; Am. Jour. Sci. Arts, CXXXV, pp. 9A-96, fig. 1-3, 1888. 
