310 



PALAEONTOLOGY OF NEW-YORK. 



propagated ; axid this trik bite, when described by European authors, brings together as its 

 localities the names of Lockport, Madison, Steuben, Cazenovia and Rochester, which, to those 

 knowing the geological position of the places, shows a species ranging from the base of the 

 Upper Silurian to the uppermost Devonian strata. The Hornalonotus {Bipleura) dekayii is now 

 known to most American students in geology to be limited in its range to the Hamilton group, 

 Tend its principal localities in New-York are in Madison county. 



This species is readily recognized, and easily distinguished from the only other known 

 American species, by the distinct trilobation of the tail in all its stages of growth, as well as 

 by the comparatively more slender and elongated form of the body, and by the more acute 

 caudal extremity. The deep furrow in the articulations leaves the anterior portion of each one 

 appearing like an intercalated articulation ; and without some knowledge, it is not easy to 

 decide whether it belong to the wider portion before or behind it. The broad, flattened and 

 falciform terminations of the articulations of the lateral lobes are very remarkable, but they do 

 not differ sufficiently from H. dekayii to render this character important. The surface marking 

 is peculiar, appearing as if made by a triangular graver with a double edge, which has cut and 

 lifted up a minute triangular point, directed more or less backwards or towards the margin. 

 The minute characters of this marking vary in some degree upon different parts of the animal, 

 but is always conspicuous on those parts which have been little exposed to wearing actipn. 



Figs. 1, 2 & 3. Individuals of different sizes, from the smallest yet seen. 



Fig. 4. An individual of medium size, and remarkable for having the caudal extremity obtuse. 



Fig. 5. A large individual of this species, where the caudal extremity is very acute. 



Fig. 6. A small specimen rolled up. This position is very rare for this species, though quite 



common in the H. dekayii. 

 Figs. 7, 8 & 9. The head and caudal shield as they occur separately in the rock. In many 



localities this species is found in no other condition. 

 Fig. 10. The head of a large individual, with a part of the glabella broken up, and which has 



carried with it the hypostoma or clypeus, the impression of which is still visible. 

 Fig. 11. The lower side of this separated fragment, showing the clypeus. 

 Fig. 12. A lateral view of several of the articulations of the body, as shown when the individual 



is bent a little downwards in the middle. 

 Fig. 13. Several articulations still more expanded, showing their entire form. 

 Fig. 14. A portion of the surface from the caudal extremity enlarged, showing the characteristic 



little papulous elevations. 



Position and locality. The principal locality of this species is at Lockport, where large 

 numbers of specimens have been obtained. It occurs in fragments, consisting of the separated 

 cephalic and caudal shields, at Rochester, Wolcott and other places. 



