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 Hyolithes 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 faunae, 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 lioea. 



n.ATE XXXII. FIGS. 11-16. 



Theca llgea. Hall. Descriptions of New Species of Fossils, etc., p. 34. 1861. 

 Fifteenth Hep. N. Y. State Cab. Nat. Hist., p. 62. 1862. 

 Hyoliiketligea, " 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 



