G. JB. Matthew — New genus of Cambrian Pteropods. 293 



increase and, ceteris paribus, loftier volcanic cones would result. 

 If, therefore, the material of the volcanic cones on the moon 

 closely resembles that of those on the earth, the enormous 

 height of lunar volcanoes is in part ascribable to the feebleness 

 of the attraction which the moon exerts upon bodies at its sur- 

 face. On the other hand, studies of the form and dimensions 



x 



of lunar volcanoes would lead to values of -, from which it 



P 



might be determined approximately whether the height of a 



column of lunar lava which would strain its lower surface to 

 the elastic limit does or does not correspond to that of columns 

 of terrestrial lavas ; a step of some interest in lunar physical 

 geology, and one which might even lead to a guess as to the 

 lithological character of the lunar rock, since different lavas 



K 



probably have different characteristic values of -. 

 Office U. S. Geol. Survey, San Francisco, Feb., 1885. 



Art. XXXYIII. — Notice of a new genus of Pteropods from the 

 Saint John Group (Cambrian) \ by Geo. F. Matthew. 



In studying the organic remains of the St. John Group the 

 writer has met with a new genus of Pteropods which is of 

 interest as showing the relation of the ancient genus Hyolithes 

 and its allies to the Cephalopods. 



Bichwald's genus Hyolithes was based on a species which is 

 camerated near the apex ; but the new genus not only has this 

 chambered area near the apex, but is also divided by dia- 

 phragms on one side of the shell, nearly to the aperture, some- 

 what in the manner of Phragmotheca of Barrande. It may be 

 described in the following terms : — 



Diplotheca n. gen. 



Slender oval cones somewhat triangular in section with abbrevi- 

 ated or attenuated apices. In the narrower part of the tube or cone 

 there are several septa that divide off segments of the lube from the 

 body cavity (chamber of habitation). The body cavity is separated 

 from one side of the outer shell by a thin partition supported by 

 delicate transverse septa or diaphragms. The apex in one species 

 is prolonged into a narrow attenuated flexible tubule with trans- 

 verse ambulations (diaphragms f) at regular intervals. 



This genus differs from Camerotheca* (another group of 

 Hyalilhoid shells associated with it in the same measures) in 

 the more rapid enlargement of the shell during growth (which 

 * Described in the Canadian Record (Canadian Naturalist), 1885. 



