266 PROCEEDINGS OF THE ACADEMY OF 



stead of being placed about half tbeir own length in advance of the posterior 

 margin of the buckler, are rather more than their entire length from the pos- 

 terior margin. The produced spine-like appendages of its cheeks are also, 

 in all our specimens, uniformly distinctly smaller, and only extend back to 

 about the termination of the fourth thoracic segment, instead of to the sixth, 

 as in D. caudata. On comparing the hypostoma of our species with Mr. Sal- 

 ter's excellent figures of that of Brunioh's species, it is found to present 

 marked and decided differences, which it would be tedious to go over in detail, 

 and which would scarcely be intelligible without the aid of figures.^ In the 

 ribs of the thorax we also observe differences, those of our species being more 

 distinctly deflected backwards, and more sharply produced at their outer ex- 

 tremities, particularly the posterior ones. The differences in the pygidium 

 are likewise well defined, its lateral margins forming almost a regular convex 

 arch from the antero-lateral rounded angles to the caudal projection (which 

 seems to be shorter, and is much narrower than D. caudata), instead of being 

 nearly straight, or even concave in outline, posteriorly. 



Most of these differences we have ascertained from a careful study of a 

 good series of specimens, to be constant in our species, so that they can be re- 

 lied upon as not being individual or sexual peculiarities. 



In some respects this species is probably even more nearly allied to the 

 common American D. limuturus, while in others it differs more widely. In 

 size it far exceeds the largest examples of D. limulurus we have ever^ seen, 

 while all our specimens show the difference in the obtusely rounded anterior ex- 

 tremity of the head, and the absence of a marginal rim around the middle of 

 the front to be constant. The convex outline of the lateral margins of its 

 pygidium, already mentioned, also contrasts strongly with that of D. limulurus, 

 and even the largest specimens of our species, five inches in length, only show 

 twelve to thirteen segments in the mesial lobe, instead of fifteen, as in the New 

 York species. The caudal appendage, if produced at all, must also be much 

 narrower at its origin in our species. 



The greater number of segments in the mesial and lateral lobes of the pygidi- 

 um, and the distinct granular surface of both D. plturoptyx and D. micrurus, 

 will alone serve to distinguish them from the species under consideration : 

 while the hypostoma of D. micrurus, at least, is entirely different. 



If the name Dalmania cannot be retained for this genus, in consequence of 

 its having been previously used for a genus of Dlptera, Hawle and Corda's 

 name Odonlocheile will probably have to be adopted for it, in which case thi$ 

 species will have to be called Odontocheile Dana. 



Locality and position. Two miles above Thebes, Alexander County, Illinois. 

 Upper Silurian. 



Genus LICHAS, Dalman, 1827- 



LlCHAS CUCDLLUS, M. & W. 



Glabella very convex ; middle lobe strongly elevated, or subconic, nearly 

 three times as wide anteriorly (measuring around the front) as behind, slop- 

 in" abruptly from the highest point behind the middle, with a straight, or 

 slightly concave outline, back to the neck furrow, and rounding with a regu- 

 lar convex, rapidly descending curve, to the rounded front; lateral slopes de- 

 clining abruptly, and separated from the lateral lobes by a linear but well 

 defined furrow, arching forward from the neck furrow, and curving laterally 

 on the anterior slope. ' Lateral lobes about half as high, and three-fourths as 

 long as the middle one, from which they slope abruptly outwards ; nearly as 

 wide behind as the posterior extremity of the middle lobe at the neck furrow, 

 but not more than half its breadth at the summit, and less than one-third its 

 anterior breadth. Outside of these, on each side, the much smaller and low- 

 er palpebral lobes are separated from them by a linear furrow, similar and 

 nearly parallel to those separating the lateral lobes from the central one. 



[Dec. 



