DESCRIPTIONS OF SPECIES. 451 



the same trend, is not always an eas}' one to maintain in practice with fossils, 

 and when the vicissitudes of preservation are taken into the reckoning it seems that 

 to distinguish the third type of surface from the second would be to rob the classifi- 

 cation of much of its utility. Therefore I am at present dispose*! to refer species 

 whose surface is smooth or marked simply l)y growth lines to Plagio(/ly2yta. 



DENTALIUM (sensu stricto) Linngeus, 1740. 



The species from American Carboniferous rocks which belong here ai'e three 

 in number. They are Dentaliura aciotisulcatum, Dentaliuni missouriense^ and 

 Dentaliurii svblseve. 



Dentalium subl^ve Hall. 



1858. Dentalium ohsoletmn. Hall, (.ieol. Surv. Iowa, vol. 1, pt. 2, p. 724, pi. 29, figs. 16, 17, 17a. 



Coal Measures. 

 1877. Dentalium siiblieve. Hall, Miller's Am. Pal. Foss., p. 244. 

 1891. Dentalium sublseve. Keyes, Acad. Nat. Sci. Philadelphia, Proc, p. 252. 



Lower Coal Measures: Des Moines, Iowa. 



Threedifferent localities have furnished each a poor specimen of a Dentalium which 

 l"jrobably all belong to the same species. I have identified them M'ith some hesitancy 

 y^iih. JDentaJ inij( xuMtrn'. Hall's brief description leaves the reader in ignorance of 

 many characters of prime impoi'tance, and his figures yield but little additional to his 

 description. The surface represented h\ the principal figures seems to be ci'ossed 

 bj" a large number of fine, clo.selv arranged, longitudinal strife, but the enlarged orna- 

 mentation shown by him consists of sharp, slender ridges, separated hj intervals two 

 or three times their own width. If the enlargement is veracious, the Colorado form 

 is certainly not Dentaliuni suMivri', which is then probably the same as D. acutisiil- 

 catum, described not long since by Gurley.** 



The shell from Colorado is rather large and massive. It has a maximum diam- 

 eter of about 7 millimeter, tapers gradually, and is slightly curved. The surface is 

 marked b}' numei'ous rather closelj' set, not verj'' prominent, ribs, which are separa- 

 ted by shallow grooves of about the same width as the stria;. There seem to be 

 about 10-15 striiu in a space of 5 mm. Rather strong oblique growth lines are 

 also visible. 



This species is possible' the same as one which occurs in the Cisco division of the 

 Carboniferous, near Graham, Young County, Tex. The two species are very simi- 

 lar, but the Texas form is practically straight, while that from Colorado is slightly 

 bent. The Texas form, when mature, becomes large and heavy, attaining a length of 

 some 250 or 300 mm., and is the one mentioned below as similar to Dentalium sp. 



oNew Carb. Foss., Bull. No. 1, p. 7, 1883. 



