32 PALAEONTOLOGY OF KENTUCKY. 



sions of the attached valve are sometimes convex, in other species, deeply 

 excavated ; those of the upper valve are usually convex. In C. tripartita of 

 Minister, the nasal process divides the fixed valve into three cells. Some of 

 the species are either entirely free or but slightly attached. Crania craniolaris 

 is the type of the genus, which extends from the Lower Silurian to the present 

 day. 



Crania bordeni. HALL AND WHITFIKLD. 



Plate II., Pig. 14. 



Crania bordeni, H. & W. 24th Regent's Report, p. 1871872. 

 Crania bordeni, H. & W. 27th Regent's Report, pi. 91875. 



Shell depressed conical, about half as high as wide ; beak sub-central, 

 slightly nearer the anterior end. Surface marked by fine radiating striae, 

 and somewhat strong lines of growth, giving a rugose character to surface, 

 especially toward the margin. This species resembles Crania crenistria, from 

 the Hamilton group of New York, but that species is more coarsely striated 

 than our shell. 



Formation and Locality. Found in the rotten hornstone and in the cherty layers on top of the 

 hydraulic limestone of the Devonian formation in Jefferson county, Kentucky, and in Clark county, In- 

 diana. The quarries for hydraulic cement rock at Watson's Station, on the Ohio and Mississippi Railroad, 

 furnished the fine specimen of Modiomorpha concentrica, to which two fine individuals of Crania bordeni 

 are attached. 



Genus Discina. 



Lamarck. 



Lamarck. Hist. Nat. des Animaux sans vertebres. 1819. Etymology: discus, a flat 

 round plate; the termination inui implying resemblance. 



The following are some of the exterior characters of the shells of this genus, 

 as given by Mr. Davidson: "Circular, longitudinally and transversely oval. 

 Upper or dorsal valve conical, patelliform, with apex inclining towards the 

 anterior margin. Lower or ventral valve opercular, flat or partly convex, and 

 perforated by a narrow oval longitudinal slit, reaching to near the posterior 

 margin, and placed in the middle of an oval depressed disk. Surface smooth, 

 ornamented by numerous striae, radiating from the apex to the margin, or by 

 concentric lines of growth produced in foliaceous expansions. Shell structure 

 horny, and perforated by minute tubuli." 



Discina doria. HALL. 



Discina doria, Hall. Pal. N. Y., Vol. IV., page 191867. 



Shell very small, sub-circular or oblate, the transverse diameter usually the 

 greater, but in some specimens it is reverse. Dorsal valve moderately convex, 



