1893.] NEW YORK ACADEMY OF SCIENCES. 71 



Mylodoma, hitherto known only from detached dental plates, 

 as described by Newberry in 1883,-^ and subsequently with 

 figures in 1882, f 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 HomoHteus. 

 On the other hand it presents evident points of ajffinity with 

 Trachosteus. The present material allows a partial determina- 

 tion of head-plates, dentition, median and lateral dorsals, and 

 ot 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 iDosition of Goccosleus among Arthrodira. 



The contribution was discussed by Dr. N. L. Bmtton. 



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 



*TransactionsN. Y^. Acad. Sci. Vol.11. 147, 

 t Monograph XVI., U. S. Geol. Survey. 163. 



