GASTEROPODS. 201 



specific titles : T. nodosa M. & W,, which was regarded as the 

 type of the genus; T hollidayi M.& W.and T. nodulosa Wor- 

 then. Now the known shells of this type have a wide geo- 

 graphic distribution. A careful comparison of a considerable 

 number of shells from widely separated localities leads to 'the 

 conclusion that the described forms of Trachydomia should 

 all be referred to only two species the earliest noticed-^T. 

 wheeleri and T. nodosa. The various slight modifications in 

 size and number of n odes are ascribed to local differences in 

 environment ; and the complete intergradation of the several 

 forms renders this view necessary. 



The callosity of the inner lip becomes very much thickened 

 in some individuals, but this feature is not so conspicuous in 

 the majority of examples. It is interesting to note in this con- 

 nection that among the shells from Illinois there are a number in 

 which the coloration of the callous portions and of the interior 

 surface is still preserved. In some cases the color is an in- 

 tense shining black ; in some a purplish black ; in others dull, 

 faded purple, and in a few the color has nearly disappeared. 

 Aside from the apertural parts all traces of the original colora- 

 tion of the shell are lost. Under the ordinary circumstances 

 of fossilization the primitive coloring could hardly be expected 

 to be preserved, except in rare cases, and the few recorded 

 instances of Paleozoic species retaining traces of the early 

 color are of peculiar interest. 



Trachydomia nodosum (MEI-K & WORTHEN). 



Naticopsis nodosa Meek & Worthen, 1860 : Proc. Acad. Nat. Sci.,Phila., 



p 463. 

 Naticopais hollidayi Meek & Worthen, 1860: Proc. Acad. Nat. Sci,, Phila., 



p. 463. 

 Naticopsis nodosa Meek & Worthen, 1866: Geol. Sur. Illinois, vol. II, p. 



366, pi. xxxi, flg. 2. 

 Naticopsis nodosa, var. hollidayi Meek & Worthen, 1866: Geol. Sur. Illinois, 



vol. II, p. 367, pi. xxxi, fig. 3. 



Shell similar to T. wheeleri^ but much larger, more massive, 

 and with the nodes of greater size. 



Horizon and localities. Upper Carboniferous, Upper Coal 

 Measures: Kansas City. 



