MICROCERAMUS. 153 



the slender, imperforate internal column of the shell; all 

 these being characters separating Microceramus from the 

 Continental groups Holospira, Epirobia, and the typical 

 Eucalodiina. What we now know seems to indicate that 

 Microceramus is an unspecialized member of the primitive 

 Antillean Urocoptine stock. 



The nucleus in Microceramus is composed of two whorls, 

 the initial half whorl smooth, the rest strongly ribbed radi- 

 ally, as shown in fig. 6 of pi. 15. At the beginning of the 

 post-nuclear growth the whorl at once widens. 



The distribution of the group indicates western Cuba as 

 the place of its origin. In the late tertiary a single species, 

 the ancestor of the M. gossei group, became widely spread, 

 and gave rise to the group of slightly differentiated races in- 

 habiting Haiti, Jamaica, across Yucatan channel to Mexico, 

 over Florida strait to Florida, etc. This particular form 

 evidently possesses exceptional means of transportation. 



Key to Subgenera and Groups of Microceramus. 



I. Axis slender and straight (MICROCERAMUS s. str.). 



1. Imperforate or very shortly rimate, th& columellar 

 margin not built forward from the columella 

 proper, or but slightly so; suture not distinctly or 

 regularly crenulate. Cuban species. 



a. Marked with opaque white on a brownish- 

 corneous ground. 



Group of M. elegans, species no. 10-15. 



5. Opaque whitish, usually with a series of spots 



above periphery and suture; no beads or 



papillae above the latter; acutely conic, the 



diam. more than half the length. 



Group of M. costellaris, sp. no. 16-19. 



2. Shortly rimate, the columellar margin a little built 

 forward; cylindric-turrite or conic-turrite, the diam. 

 less than half the length; suture usually denticulate 

 or crenate. Antilles and mainland of America. 



Group of M. gossei, sp. no. 1-9. 



