58 



one of the cheeks, and a portion of the front are ob- 

 scured by the rock in which the animal is imbedded. 

 The cheeks form spherical triangles. The oculife- 

 rous tubercles, though a good deal defaced, seem to 

 have been circular and not lunate, as in the A. Cau- 

 datus. A raised, curved line passes from and over the 

 eye, between it and the lateral lobe of the abdomen. 

 The central lobe of the back is composed of twelve 

 double joints, and that of the tail of six single articula- 

 tions; where the epidermis or shell is perfect, all the 

 articulations appear single. The last joint of the tail 

 is longer than in any other of our species. The ribs 

 of the abdomen are rather broad, and have a deep 

 furrow scooped out along their upper surface; their 

 extremities, where they can be discovered, are de- 

 tached from each other, and terminate in reflected 

 points, like those of the A. Limulurus. The costal 

 arches of the tail are delicately grooved, and termi- 

 nate in the membrane. The membranaceous expan- 

 sion round the edge of the tail is very narrow, and 

 appears to form no projection beyond its central 

 part. The whole epidermis is finely marked with 

 granulations. Length one inch and three-fourths 

 breadth one inch and one-fourth. 



This interesting species was found in limestone 

 shale, near Rochester, in Munroe County, N. Y.j and 

 is now in the valuable cabinet of the Albany Institute. 

 An accidental fissure of the rock disclosed not only a 

 fine specimen of both the mould and the cast of this 

 animal, but also another individual of the same spe- 

 cies in contact with it. From the peculiar attitude 



