338 G. If. (Urty — G rowth Stages in Naticopsis altonensis. 



A bt. XXXIV. — I. On Some Growth Stages in Naticopsis 

 altonensis, McChesriey* y by George II. Girty. With 

 Plate I, tigs. 1-8. 



The object of this paper is to call attention to certain strik- 

 ing changes in shape and sculpture shown in the development 

 of one of our common Pennsylvanian species of JSTaticopsis, 

 representing stages so distinct that they might be easily mis- 

 taken for different species, although to my knowledge no such 

 mistake can be cited. It is possible, however, from the material 

 at hand to connect all these aspects with a single specific type. 



The species in question is familiar to everyone, though no 

 one can be quite sure, as a matter of synonymy, just what 

 name to call it by. As Naticopsis altonensis it is perhaps as 

 familiar as by any other designation. 



A chief feature of the mature form is its elongated shape 

 and the strong deflection of the surface which takes place 

 between the periphery and the suture and gives the final volu- 

 tion a strongly sinuous outline. The upper portion of this 

 volution is marked by rather strong regular plications extend- 

 ing downward from the suture with a slight backward slope. 



In following the mature form to its more youthful stages, 1 

 find that the lateral sinus first disappears and then the sub- 

 sutural plications, so that we have in the beginning an unorna- 

 mented shell with regularly formed whorls, then an ornamented 

 shell with regularly formed whorls, and lastly an ornamented 

 shell with sinuated whorls. 



The first condition is maintained until about four volutions 

 have been formed, and a height of 7 mm attained. The shape 

 is rather elongated and the spire relatively high for the genus. 

 The volutions are regularly rounded and marked only by fine 

 incremental lines. Some specimens, however, show also 

 extremely delicate revolving lines, so fine and obscure as to 

 suo-aest their origin in a fibrous structure of the shell rather 

 than in a system of sculpture. 



The second condition occupies scarcely more than a single 

 volution. The plications or fasciculated growth-lines below 

 the suture are introduced gradually, but are rather rapidly 

 developed. A specimen exemplifying this stage completed has 

 a height of about ll mm . 



The final condition, in which is introduced the second 



feature of the mature shell, the lateral sinus, occupies two or 



more volutions, the enlargement being rapid (especially as 



to length), so that the dimension must have been 55 mm or 



* Published by permission of the Director of the U. S. Geological Survey. 



