1896.] NEW YORK ACADEMY OF SCIENCES. 57 



YI. The Fuess Goniometer as am Axial Angle Apparatus. 



By the addition of a double convex lense and two nicols 

 prisms the No. 2 Fuess Goniometer can be converted into an 

 exceedingly- convenient apparatus for the measurement of the 

 angle between the optic axes. 



One nicol replaces the signal tube in the collimator, the other 

 replaces the eye piece in the observation telescope, both are ad- 

 justable and the one in the collimator is graduated for accurate 

 adjustment. 



The tube containing the condensing lens fits over the objec- 

 tive end of the collimator like a cap and is held firmly in place 

 by three clamp screws. The observation telescope is converted 

 into a weak microscope by dropping the extra lens ordinaril^^ 

 used to bring a ciystal into focus. 



The crystal section is carefuUv mounted with the plane of the 

 previousl}^ determined optic axes horizontal. By inserting a 

 signal tube the observation telescope is set opposite the colli- 

 mator. The signal tube is removed, the two nicols and condens- 

 ing lens adjusted as described the nicols being crossed 45° to 

 the plane of rotation of the goniometer and rotation of the care- 

 fully centred crystal brings the two branches of the hyperbola 

 successively into contact with the vertical cross hair. 



NOTE ON THE VENTRAL ARMORING OF 

 DINICHTHYS. 



By Bashford Dean, 



Department of Zoology, Columbia University. 



In a recent paper * by the present writer a partial description 

 was given of the A-ertebral column, fin supportsand ventral armor- 

 ing of Dinichthys goiddii(J). This account had been based upon a 

 unique specimen which had been discovered by Dr. William 

 Clark in the Cleveland shale f of Rocky River, near Berea, 

 Ohio, and had been courteously loaned the writer for examina- 

 tion. It has since that time, thanks to the generositj'^ of Dr. 



♦Trans. N. Y. Acad. Sciences. May 11, ls%. 



fDr. Clark notes that the reader might have inferred from the passage, op. cit., p. 

 158, that the concretion had been collected in Berea sandstone. " This unique speci- 

 men was discovered in the vicinity of Berea, * * * in a shaly concretion, somewhat 

 coarser in texture— sandstone-like and lighter in color than those commonly found 

 in the Cleveland shales." Dr. Clark notes in a recent letter " the concretions in which 

 the fish are found differ materially from each other: they «re never the same in the 

 Cuyahoga valley as in the Rocky river region, in the latter we find them as in the one 

 you had (the one in question), or in ' cone and cone.' " 



