268 STUDIES IN EVOLUTION 



or one alone may be present. In case both appear, the 

 Paterina stage is always the first one to be developed. 



The nepionic stage of Leptcena rhomboidalis (figure 7, 

 Plate XII) is represented by a shell without radii, having 

 a comjiaratively large pedicle-opening in the ventral valve 

 and a large deltidium. The hinge is not well defined and 

 the shell is discinoid in form. This term is not used to 

 suggest any special affinities with true discinoid genera, as 

 Orhiculoidea or Discinisea. The proper name for this stage 

 is not yet apparent to the writer. Tlie external characters 

 as expressed by both valves are manifestly nearer to Kutor- 

 gina than to any telotremate genus. Until the early forms 

 belonging to the articulate brachiojDods, especially to the 

 orthoid and strophomenoid groups, have been thoroughly 

 studied, the interpretation of the nepionic Leptcena rhom- 

 boidalis may be uncertain. It should be noted, however, 

 that the young of Chonetes, Productus, Stropheodonta, Oriho- 

 tltftes^ Leptcena., Plectambonites, and Strophomena, all have 

 little or no indication of a straight hinge-line, and that the 

 extension of this member takes place during later neanic 

 and ephebic growth. This in itself is significant, but is 

 more marked when taken with the growth-stages shown by 

 some species of Strophomena which have after the protegu- 

 lum a Paterina-\\kQ stage, with a straight hinge in the dorsal 

 valve, succeeded by holoperipheral, discinoid, nepionic growth, 

 and finally a renewal of a straight-hinged condition. Thus it 

 has an early straight-hinged form, which is lost during the 

 next stage of growth, and again appears, and is progressively 

 elongated during neanic and ephebic growth. 



The nepionic stages of Terebratulina septentrionalis (figure 

 10, Plate XII) represent a decreasing extension of the cardi- 

 nal line from the protegulum, an open delthyrium, the 

 absence of radii, and the introduction of the shell punctae. 

 The crura at this stage, as shown by Morse, are short and 

 stout, and the loop is undeveloped. 



Neanic Period. — During the progress of this period all 



