264 STUDIES IN EVOLUTION 



line, and is eminently a primitive character in the Pro- 

 tremata. 



From present knowledge of the group it is difficult to offer 

 an explanation for the presence of an anal opening in the 

 Inarticulata and its absence in the recent Artie ulata, as the 

 solution of the question depends upon whether the class is to 

 be considered as progressive or degraded. The dorsal beaks 

 of Amphigenia.) Athyris, Cleiothyris, Atrypa, and RJiyncho- 

 nella are usually notched or perforate. The perforation 

 comes from the union of the crural plates above the floor 

 of the beak leaving a passage through to the apex. A 

 similar opening occurs between the cardinal processes in 

 Strophomena, Stropheodonta, and allied genera, and the chi- 

 lidium may also be furrowed, as in Leptcena rhomboidalis. 

 This character is evidently in no way connected with the 

 pedicle-opening, but points to the existence, in the early artic- 

 ulate genera, of an anal opening dorsal to the axial line, as in 

 the recent Crania. This dorsal foramen was described and fig- 

 ured by King 13 in 1850, Hall 8 in 1860, and by several authors 

 since, and has commonly been termed a visceral foramen. 



GEhlert 21 suggests that it was probably occupied by the 

 terminal portion of the intestine. The persistence of the 

 foramen seems to indicate an anal opening. In reference to 

 this character and the obsolescence of the eyes the class 

 must be viewed as retrogressive since Paleozoic time. Other 

 features, however, are manifestly progressive; namely, the 

 gradual shortening, through time, of the posterior elements 

 of the animal, as the pedicle, visceral portions, and internal 

 shell structures, and the expansion of the anterior parts, as 

 the shell and brachia. 



A further advance in specialization is shown in the limita- 

 tion of the pedicle -opening wholly to the ventral valve in the 

 higher rhynchonelloids, athyroids, spiriferoids, and terebratu- 

 loids. The absence of punctse in all the early radicles and 

 their subsequent development in the derived types may also 

 have a similar bearing. 



