Silurian Fossils. 29 



The spondylium of the pedicel valve encloses a long, nar- 

 row, and deep cavity extending to within one-third of the 

 length of the shell from the anterior margin. This spondylium 

 is supported by a median septum extending almost to the 

 anterior edge of the valve. 



The interior of the brachial valve is marked by two sharp 

 septal ridges, only moderately divergent anteriorly, extend- 

 ing fully a third of the length of the shell from the beak. 

 Near the anterior part of the space between these septal 

 ridges, a third septal ridge begins, extending to within a 

 third of the length of the valve from the anterior margin. 

 Posteriorly, the two septal ridges first mentioned become ele- 

 vated into septal plates, supporting the crural plates. In a 

 specimen, etched by means of acid, the top of the septal 

 plates curves smoothly into the crural plates, there being no 

 evidence of the former extension of the inner margins of 

 the crural plates beyond the line of junction with the septal 

 plates. The exterior margin of the crural plates defines the 

 inner margin of the dental sockets. Enclosed by the two 

 septse, the crural plates, the remainder of the hinge plate, 

 and the posterior part of the valve are two cavities, one on 

 each side of the shell. Immediately in front of the beak, 

 there is a thickening of the posterior part of the crural plates, 

 extending in a semi-lunate fashion around the posterior ex- 

 tremity of the depression limited by the septal plates already 

 described. This semi-lunate thickening, indistinctly defined 

 posteriorly, apparently is part of the cardinal process. 



Kokomo limestone, at Kokomo, Indiana, near the top of 

 the exposures at the Defenbaugh and Traction line quarries. 



Compared with Pcntanierus pcs-ovis, Whitfield, from the 

 Greenfield limestone in Adams county, Ohio, the beak of 

 the pedicel valve is less incurved at its extremity, and the 

 septal plates of the brachial valve are less distant from each 

 other. Moreover, the size of the Kokomo species is distinctly 

 larger, although apparently it is closely related to the Green- 

 field limestone specimens described by Whitfield. 



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