200 GASTEROPODS. 



and Naticopsis nodosa Meek & Worthen seem to be refera- 

 ble to Trachydomia; the other three forms described under 

 the genus M. & W., T. hollidayi M. &. W., and T. nodulosa 

 Worthen, being at present regarded merely as more mature 

 individuals the first of T. nodosa, and the second of Swallow's 

 species. A comparison of an extensive series shows that within 

 certain limits the shells of this group are quite variable. The 

 callosity in some specimens is much more extended than in 

 others ; while numerically the surface nodes vary greatly in 

 different individuals, and become much larger and more widely 

 separated as, with age, the shell increases in size. 



Trachydomia wheeleri (SWALLOW). 



Plate lv, fig. 8. 



Littorina wheeleri Swallow, 1860: Trans. St. Louis Acad. Sci., vol. 1, p. 658. 



Naticopsis (Trachydomia} wheeleri Meek & Worthen, 1866: Geol. Sur. Illi- 

 nois, vol. II, p. 364. 



Naticopsis wheeleri Meek & Worthen, 1873: Geol. Sur. Illinois, vol. V, p. 

 595, pi. xxviii, fig. 3. 



Naticopsis wheeleri White, 1884: Geol. Sur. Indiana, Ann. Rep. 1883, pt. 

 ii, p. 162, pi. xxxii, figs. 13-14. 



Trachydomia nodulosa Worthen, 1884: Illinois State Mus. Nat. Hist., 

 Bui. 2, p. 8. 



Trachydomia wheeleri Keyes, 1889: Am. Geologist, vol. IV, p. 195. 



Trachydomia wheeleri Keyes, 1890: The Nautilus, vol. IV, p. 30. 



Trachydomia nodulosa Worthen, 1891: Geol. Sur. Illinois, vol. VIII, p. 

 146, pi xx iii, figs. 11-lla. 



Trachydomia wheeleri Keyes, 1891: Proc. Acad. Nat. Sci., Phila., p. 257. 



Shell thick ; whorls four or five ; the spire rather elevated, 

 aperture ovate, outer lip rapidly becoming attenuated; callosity 

 of the inner labrum thick and greatly extended. Oolumella 

 heavy. Surface covered by numerous regularly arranged tu- 

 bercles. 



Horizon and localities. Upper Carboniferous, Coal Meas- 

 ures : Clinton (Henry county), Kansas City. Also Peoria, 

 Springfield, Alton and elsewhere in Illinois ; Des Moines, Iowa; 

 and, according to White, in New Mexico. 



The first species of this group described from the Ameri- 

 can Paleozoic rocks was brought to notice by Swallow under 

 the name of Littorina wheeleri* Since the appearance of Swal- 

 low's description, three other similar shells have been given 



