Report of the State Geologist, 585 



The cardinal area is about equally developed on each valve. 

 Surface ornamented by fine, rounded, radiating striae between 

 each two of which are from two to six much finer radiating 

 lines; all these are crenulated by exceedingly faint growth-lines. 



The interior of the pedicle-valve has short teeth with prominent 

 extremities, and inconspicuous dental lamellae. The muscular 

 impression is large and consists of a small adductor scar situated 

 centrally, and two Lateral scars which are Habellate and greatly 

 elongated, hut not uniting in front and inclosing the adductor, as 

 is usual in typical forms of Orthis. These lateral scars are 

 divided into subordinate impressions, of which the outer posterior 

 members may represent the adjustors. In the brachial valve the 

 dental sockets are obscure, the crural plates oblique, terminating 

 abruptly at the bases of the crura, their lower part being con-' 

 tinued in a low ridge surrounding a short subcircular muscular 

 area, which is very obscurely quadrilobate. The cardinal process 

 has a vertical, sharp, simple, posterior edge, but is much thickened 

 where it unites with the crural plates, and is produced along the 

 muscular area as a prominent median ridge. From the ante- 

 lateral margins of the muscular area radiate six low, somewhat 

 sinuous ridges of similar character to those frequently seen in 

 members of Group XL Both valves are considerably thickened 

 just within the margins, as in many strophomenoids. 



The shell-structure is finely fibrous and perforated by minute 

 punctations which are in general sparsely developed, but most 

 distinctly arranged in radiating rows corresponding to the 

 surface striae. 



(Lower Silurian.) 



Bilobites, Linne. 17T5. 



Synonym : Diccelosia, King. 1850. 



IX. Group of Orthis biloba, Linne. 



(Plate 10, Figs. 14^19.) 



Shells small, strongly bilobed. Hinge-line short; teeth and 

 sockets obscure ; cardinal process small and simple. Crural plates 

 thin and very long, often extending as far forward as the com- 

 mencement of the lobation. Muscular area indistinct, larger in 

 the^ brachial, than in the pedicle- valve. Shell-substance fibrous 

 1S91. 74 137 



