1893. ] NEW YORK ACADEMY OF SUIENCES. ae 
Mylostoma, hitherto known only from detached dental plates, 
as described by Newberry in 1883,* and subsequently with 
figures in 1882,{ now proves, as was conjectured, an armored 
form. Its plates are slighter in character than those of kindred 
American forms, do not exhibit sculpturing, and in arrange- 
ment, as well as size, approach those of the European Homosleus. 
On the other hand it presents evident points of affinity with 
Trachosteus. The present material allows a partial determina- 
tion of head-plates, dentition, median and lateral dorsals, and 
of abdominal shield. As usual with Arthrodiran remains, no 
traces are to be found of visceral arches or of fin structures. It 
would appear, in summary, that in this form there is a new 
example and an extreme one of specialization in Arthrodira. 
In this case the modification has extended to dental plates 
adapted for grinding, and in all probability to a body depressed 
dorso-ventrally, with eyes placed dorsally and somewhat closely 
together, characters which perhaps might be expected in a 
fish of ray-like habits. 
The relations of the Waverly fish fauna, as of salt water 
origin, to that now known in Belgium, as shown by Lohest, was 
commented upon, and a brief discussion was given regarding 
the position of Coccosleus among Arthrodira. 
The contribution was discussed by Dr. N. L. Brirron, 
ON AN OCCURRENCE OF GABBRO (NORITE), NEAR 
VAN ARTSDALEN’S QUARRY, BUCKS COUNTY, 
PENNSYLVANIA, 
BY J. F. KEMP, 
In the winter of 1886 the writer, then abroad, received from 
Dr. N. L. Britton a series of rocks, that he had collected the 
previous summer while doing field-work for the New Jersey 
Survey. Among them was one from the State collections, 
marked Van Artsdalen’s Quarry, Bucks County, Pennsylvania. 
On microscopic examination it proved to be a norite of singu- 
larly fresh condition and of typical mineralogical composition. 
The interesting, diagnostic mineral hypersthene exhibited its 
pink and green pleochroism with great vividness, while with it 
were a light green, monoclinic pyroxene, considerable yellowish- 
brown hornblende, plagioclase and magnetite. The rock 
a 
*Transactions N. Y. Acad. Sei. Vol. II. 147. 
t Monograph XVI., U.8. Geol. Survey, 163. 
