236 PALAEONTOLOGY OF NEW-YORK. 



lateral lobe ; pygidium composed of two or three articulations above, with a fourth and 

 last which is very long, and abruptly narrowing towards the extremity ; lateral lobes with 

 three articulations on each side, which are marked by a narrow groove for about one third 

 of their length ; central lobe separated from the lateral lobe by a deep narrow groove. 



The specimens which we possess are too imperfect to give a complete description of the 

 fossil. Two of these are from Carlisle ( Pa.) , being the same figured by Mr. Conrad as 

 cited above. It is certainly quite distinct from Asaphus, and clearly referable to Platynotus, 

 when compared with specimens from Lockport, which have the posterior part of the glabella 

 much narrower proportionally, and the entire fossil is less convex. The Lockport species 

 likewise difl'ers in the anterior extension of the buckler, which is not shown in our speci- 

 mens, or in those figured by Capt. Portlock. The articulations of the thorax differ in their 

 character ; and tlie posterior extension of the middle lobe of the caudal shield is much 

 narrower in the Trenton species, than in the other. This feature is shown in our figures, 

 as well as in those of Portlock before referred to. 



Emmrich, in his description of Nuttainia, quotes JY. hibernica as the typical form, 

 remarking that " a perfectly corresponding species occurs at Lockport in North America." 

 I have already remarked that the Lockport species is distinct from the one under con- 

 sideration, as well as from the figures of Portlock ; and the specimen referred to must be 

 either distinct, or not from Lockport. 



Fig. 1 a, b. Portions of the buckler and caudal shield of this species. 



Fig. 1 c. A fragment of the buckler, more compressed than the preceding. 



Fig. 1 d. A magnified portion of the crust, showing the pustules upon the surface. 



Fig. 1 e. This figure is from a plaster cast of a specimen, the original of which is in the cabinet of Mr. 

 Carly of Cincinnati, The specimen is from the Blue limestone of Ohio, and is apparently 

 identical with our species. The oculiform tubercles are well preserved, but their true cha- 

 racter cannot be learned from a cast. 



Position and locality. This species, so far as known, is confined to the Trenton limestone, 

 or rocks of the same period. The specimens 1 a, h, are from Carlisle (Pa.), where they 

 are associated with other Trenton limestone fossils. Fig. 1 c is from Middleville ( N. Y.). 

 Fig. 1 e is from a corresponding position at Cincinnati (Ohio). 



It is an interesting fact, tliat species of this genus appear in three distinct positions in the 

 Silurian strata ; the first in tjie Trenton limestone, the second in the Niagara group, and 

 the tliird in tlic Delthyris shaly limestone, which we must regard as near the termination 

 of the Silurian period. 



