72 PALAEONTOLOGY OF NEW YORK. 



tin' axis. The three pairs of lateral spines are curved regularly backward, 

 while the four posterior spines appear to have been straight, and but slightly 

 divergent. Within the margin, and just above the bases of the long third 

 pair of spines, was another pair which, judging from the diameter of the 

 broken base, must have had fully the length of the third pair. These spines 

 were inclined at an angle of nearly forty-five degrees to the surface of the 

 border. The actual length attained by all these spines is not indicated except 

 for the third pair: judging, however, from their broken sections it is evident 

 that they were of very considerable length. Across the posterior border the 

 bases of the spines art' continued as low ridges to the edge of the axis. 



Surfaci Ornamentation. The bulbiform termination of the axis is covered 

 with closely set pustules of uniform size. The anterior furrow and the 

 border are minutely granulose, and the surface of the spines is marked by 

 elevated, frequently interrupted striae, which converge at an acute angle on 

 the upper surface of the spine; towards the extremity of the spines these 

 lines become stronger and much rounded, giving the surface a coarsely 

 ridged appearance. 



Dimensions. The single specimen observed has a width across the anterior 

 margin of 9 mm., a length of 8.5 mm. to the posterior margin, and of 33 mm. 

 axially, as far as the apex of the third pair of spines. 



Observations. This extravagant form of Acidaspis is the only member of the 

 genus as yet known in America from rocks younger than the Upper Helder- 

 berg limestone. The great length of the caudal spines is equaled by no other 

 known species of the genus and is approximated only by Acidaspis Dufresnoyi, 

 Barrande. ' 



Distribution. Hamilton group. Little Traverse Bay, Michigan. 



We are indebted to Dr. < '. Rominger, of Ann Arbor, Michigan, for the discovery of this species, and 

 li his favor, the i been made accessible for study and description. 



