508 S. WEL1.ER MISSISSIPPIAN RHYNCHONELLIFORM SHELLS 



cavity being identical in all essentials with the similar structure in many 

 undisputed rhynchonelloid species. 



In the pedicle valve the dental lamellae are strongly developed and 

 meet near the floor of the valve to form a spondylium which is supported 

 by a median septum. Anteriorly the spondylium is progressively more 

 elevated above the floor of the valve by the median septum. For a short 

 distance, opposite the articulation of the valves, a pair of slight, lateral 

 processes are present upon the outer surfaces of the dental lamellae near 

 their attachment to the median septum, which are possibly incipient 

 lateral buttress plates such as are present in the genus Tetracamera. 



PuGNAx Hall and Clarke 



PUGNAX PUGNUS (MARTIN) 



The remaining species which will be considered here have never been 

 removed from the family Ehynchonellidse, although they have been dis- 

 tributed among several different genera, and in some cases their generic 



0988QQ 



<^ d e / 



Figure 7. — Cross-sections of the rostral Portion of Pugnax pugnus (Martin) 



This series of six cross-sections (X 2%) is from a specimen from the Mountain lime- 

 stone of Ireland 



reference has clearly been incorrect. Hall and Clarke have referred no 

 less than five Mississippian species to the genus Pugnax, and Schuchert 

 has placed the same species in the genus, the generic characters most 

 commonly depended upon being the external configuration of the shell, 

 most especially the obsolescence of the plications except toward the an- 

 terior margin of the shell. In order to have a standard for the compari- 

 son of the American species referred to this genus, one of the typical 

 species of the genus, Pugnax pugnus, from the Mountain limestone fauna 

 of Ireland, has been investigated, and the cross-sections secured are shown 

 in figure 7. The structure of this shell is exceedingly simple. No me- 

 dian septum is present in either valve; the hinge-plate of the brachial 

 valve is divided to the apex, and the dental lamellae of the pedicle valve 

 are short. None of the Mississippian species referred to the genus Pug- 

 nax by Hall and Clarke or by Schuchert possesses this simple structure, 

 and in fact no species possessing the combination of this arrangement of 



