PTEROPODA. 
195 
knowledge of the species of the primordial zone is too limited to speak with 
confidence of their geographical distribution. 
In the primordial zone, the fossils of this genus are associated with Linguloid 
and Oboloid shells, and often with Trilobites; but the most prolific Trilo- 
bite sandstones in the Mississippi Valley rarely contain them. The rare 
occurrence of Hyolithes in the second and third horizons does not admit of 
faunal comparisons, and the same may be said of those occurring in the Upper 
Helderberg group. In the Hamilton group the association of IIyolitiies is 
with numerous Brachiopoda, many Lamellibranchiata (especially of the Nucu- 
loid forms), some Gasteropoda and many Trilobites of a single species. The 
older forms occur in a nearly pure sandstone, and in calcareous sandstone, as 
well as in more argillaceous beds. The species of the second fauna occur in a 
semi-calcareous shale; and those of the third fauna in limestone. The two 
species of the Upper Helderberg group are in limestone or arenaceous limestone, 
and those of the Hamilton group in semi-calcareous shales. 
Some, at least, of the older forms are chitinous or phosphatic in their 
structure, and we have observed the same conditions in some of those of the 
primordial zone of Sweden. Those of the second and third faume, and those 
referred to Devonian age, are calcareous. In the two former the shells are 
comparatively thick and strong, and in the latter they are thin and fragile. 
Hyolithes ligea. 
PLATE XXXII, FIGS. 11-16. 
Theca ligea, Hall. Descriptions of New Species of Fossils, etc., p. 34. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist , p. 62. 1862. 
Hyolithes ligea, “ Illustrations of Devonian Fossils : Pteropoda, pi. 27, figs. 13-16. 1876. 
Form an elongate subtriangular pyramid, with the lateral margins acute, gradu¬ 
ally tapering from the base to an acute extremity. Transverse section 
somewhat semi-elliptical, with a moderate convexity on the ventral 
side, the lateral edges compressed and subacutely angular. Ventral face 
gently convex, both transversely and longitudinally. Dorsal face highly 
convex, slightly concave in a longitudinal direction, and obtusely angular 
