12 BracMopoda of the C'mchmatl Group. 



the valves and area ; structure calcareous and tubular. Iii the in- 

 terior, four principal scars formed by the adductor muscles are observ- 

 al^le. The anterior pair are approximate and placed close to the 

 center, behind -which a prominence is sometimes visible in the ventral 

 valve. The posteror pair are situated near the cardinal edge, and are 

 widely separated. The muscular impressions of the attached valve are 

 sometimes slightly convex, at others deeply excavated ; those of the 

 dorsal valve are convex, the center pair sometimes developing very 

 prominent apophyses. The interior of the attached valve is surrounded 

 by a raised and thickened border, exhibiting the tubular shell 

 structure in a cons^iicuous manner. The disk of each valve exhibits 

 more or less distinct impressions of the vascular system, which is sim- 

 ply digitated. 



Crania scahiosa — (Hall, 1866). 



Shell somewh.^t less than medium size, usually discoid or little 

 elevated, but sometimes prominent, irregular in outline ; margin thick- 

 ened, apex of dorsal valve eccentric, varying in different individuals. 

 Surface of valve having usually strong lamellose lines of growth, Avhich 

 are sometimes obscured by the roughness of the substance to which 

 the specimen is attached, showing through the shell or causing it to 

 grow irregularly, by which it often assumes the features of the foreign 

 body. Interior showing the two anterior muscular impressions situated 

 posterio-centrally, and the two posterior muscular impressions a 

 little farther separated from each other, and situated close to the car- 

 dinal edge. 



It is a common fossil throughout the Cincinnati Group, at all eleva- 

 tions above 200 feet from low water-mark. 



Crania Icelia — (Hall, 1866). 



Shell small, discoid or moderately convex on the upper valve, some- 

 what narrowed toward the cardinal border. Apex of the dorsal valve 

 minute, not prominent, situated about one third the length of the 

 valve from the cardinal margin. 



Surflice marked by fine but very sharply elevated radiating strite, 

 which are sometimes tortuous, and frequently increased by implanta- 

 tion; ventral valve and interior unknown. 



It was first found at Cincinnati, about 400 feet above low water-mark, 

 and extends from thence to near the Upper Silurian rocks. It is not, 

 however, a common fossil. 



