UROCOPTID.E. XVli 



a few small survivors. Packard has remarked of the Trilo- 

 bites, Brachiopods and Ammonites, that "these types, as is 

 well known, had their period of rise, culmination, and de- 

 cline, or extinction, and the more spiny, highly ornamented, 

 abnormal, bizarre forms appeared at or about the time when 

 the vitality of the type was apparently declining." The 

 Vrocoptina: are now apparently in a similar stage of ex- 

 travagant variation. 



PARALLEL AND CONVERGENT EVOLUTION. No one whoi 

 studies species belonging to a number of groups of Urocop- 

 tidcs can fail to be impressed by the apparent "parallelism" 

 or "convergence" in the structure of the axis of the shell, 

 in the members of genera but distantly related. To some 

 extent a similar tendency affects the dentition of the radula 

 also. Thus at least part of the specialization of the lateral 

 teeth of Brachypodella, Pineria and Tetrentodon seems to 

 have been an independent process in each of these groups, 

 as though the former two had successively seceded from an 

 ancestral stock having teeth similar to Urocoptis, while 

 Tetrentodon certainly came from Gongylostoma, a much later 

 stock than that whence the others arose. 



In the shells, homologous regions of the shell or its axis 

 have given rise to similar structures, wholly independently, 

 in various phyla of the family. Thus we find hollow ribs of 

 like structure in species of Holospira, Idiostemma, Tetren- 

 todon, Callonia, and Brachypodella groups belonging to two 

 subfamilies and several minor phyla, and in each case re- 

 lated far more nearly to forms with normal sculpture than 

 to each other. 



The axial appendages are almost interminably repetitive. 

 Among many equally available instances, the following may 

 serve as illustrations of structures similar in the three sub- 

 families : 



Axis Lamella Lamella in Axis with 



simple. throughout. later whorls. vertical ribs. 



Eucalodiince: Oligostylus. Eucalodium. Anisospira. Cielocentrum. 



UrocoptincR : Urocoptis e. s. Arangia. Spirocoptis. Idiosiemma. 



Microceramince : Microceramus. Spiroceramus. 



