BEAOHIOPODA OF THE MAEL BEDS. 15 



sometimes slightly flattened in the middle region inetead of having plica 

 tions, as in T. plicata. On some individuals the plications are all but obso- 

 lete, being very faintly marked on the central portions and obliterated 

 toward the front of the shell by the strong concentric lines and varices of 

 growth. The punctate structure of this shell is about the same as that of 

 T: plicata. 



This shell has been very generally admitted as a valid species by 

 Palaeontologists ; still I have very great doubts of its specific distinction. 

 It occurs at nearly all, if not quite all, localities where T. plicata is found, 

 but is quite rare in its typical character. Individuals can be selected very 

 readily from any of the localities where it is found that will unite by 

 insensible gradations the utmost extremes attained by the two forms. I 

 have not seen the internal features of this form to know if they agree 

 with those of T. plicata^ as specimens presenting the features of the species 

 in good degree are too rare to be sacrificed in an uncertain attempt to 

 develope them. 



Formation and locality. — Usually found associated with T, plicata^ Say, 

 at most of the localities where that one occurs, and only in the Lower Marl 

 Beds associated with Gryphcea vesicularis] Exogyra costata, etc. 



