MARYLAND GEOLOGICAL SURVEY 277 



vermiform tubes with their surface usually smooth, but sometimes appar- 

 ently striated. Their form is rigidly straight and they range in length 

 from several inches to a foot or more. The diameter varies between one- 

 eighth to one-half an inch. 



These tubes 'preserve their distinctness under almost all conditions 

 and they often stand out quite clearly in the rock. 



Occurrence. HARPERS SHALE AND ANTIETAM SANDSTONE. In Mary- 

 land this species has been identified at Eakles Mills and other localities, 

 particularly in the drift blocks along the west front of the Blue Ridge. 

 Widely distributed in the Cambrian of Pennsylvania, New York, Vir- 

 ginia, Canada, etc. 



Collections. Maryland Geological Survey, U. S. National Museum. 



order PRIONODESMACEA 



Family CTENODONTIDAE 



Genus CTENODONTA Salter 

 CTENODONTA GIBBERULA Salter 



Plate XLII, Figs. 18-20 



Ctenodonta gWberula, Salter, 1857, Canadian Org. Rem. Dec. I, p. 38, pi. viii, 



fig. 6. 

 Tellinomya ventricosa Meek and Worthen, 1868, Geol. Surv. Illinois, vol. iii, 



p. 307, pi. ii, figs. 7a-c. 

 Ctenodonta gibberula Ulrich, 1894, Geol. Minnesota, Pal., vol. iii, pt. 2, 



p. 587, pi. xlii, fig. 37, text figs. 44f-g, p. 599. 



Description. " Shell rhombic subovate, ventricose, the height, length 

 and thickness, respectively, as seven, ten, and six, with large incurved 

 beaks, situated a little behind the mid-length ; antero-dorsal and ventral 

 margins subparallel, the posterior end obliquely truncate above the narrow 

 and sharply rounded lower part; anterior end broadly rounded and con- 

 tinuing into the basal margin ; the latter is straight or very gently sinuate 

 and ascends from the prominently rounded anterior part; posterior 



