VERTEBRATES. 373 



species by its great relative breadth, its flattened, compressed sides, and strong, 

 crowded ornamentation. The number of the longitudinal ribs is not the same 

 on the two sides, and they increase toward the base, as is usual in the genus, 

 by their bifurcation. They cover the entire surface of the exposed portion, 

 except just at the summit, which is smooth, and evidently worn by use. 



Formation and locality : Lower Carb. limestone j Burlington, Iowa 



GENUS PHYSONEMUS, Ag. 



PHYSONEMUS GIGAS, N. and W. 



PI. ii, % i. 



SPINE large, massive and strong, one foot or more in length, 

 two inches in breadth, and three-fourths of an inch in thick- 

 ness at the base ; strongly curved, with the summit turned to- 

 ward the front; base expanded, flattened, and somewhat bi- 

 lobed; posterior (convex) margin slightly flattened, and ob- 

 scurely furrowed longitudinally; anterior (concave) margin 

 sub-acute, beveled, the flattened faces which include the angle 

 covered with a fine reticulated ornamentation, and bearing 

 each a single row of remote, large obliquely-stellate, enamelled 

 tubercles, which alternate with those of the other side. 



Only the basal portion of one of these spines has, at yet, come under our ob- 

 servation. This is, however, sufficient to prove it quite different from any 

 other hitherto described from an American locality. Its rarity, great size, pe- 

 culiar markings and reversed curve, all combine to make it a specimen of unu- 

 sual interest, but it has a still higher value in its close generic identity and 

 specific affinity with a spine obtained by Prof. McCoy from the Carboniferous 

 limestone of Armagh, Ireland, and described by him (Brit. Paleoz. Fossils, p. 

 638, PL 3 f,jftff- 29) under the name of Physonemus arcuatus. 



The resemblance between the spine described and figured by McCoy, and 

 that before us, is so strong that we have had some hesitation in deciding them 

 to be distinct. Our spine is, however, many times larger than that which oc- 

 curs in the Armagh limestone, and the large stellate tubercles of the anterior 

 margin are very obliquely conical, the summit being turned toward the base of 

 the spine, so that if a little prolonged they would form hooks. In Ph. arcuatus 

 they are represented as being symmetrical. No generic or specific description 



