OBSERVATIONS ON THE GENUS ATRYPA. i7i 



superinduced by a difference of conditions existing at certain localities 

 during the life of the animal. I have not yet carried the investigations 

 far enough to satisfy myself fully of the permanency of these internal 

 differences in specimens of the same type. The fact of different varie- 

 ties being often restricted to a certain geological position is, however, 

 good grounds for supposing that the peculiar internal features may also be 

 retained. 



The extreme form of the loop noticed in those from the Shaly lime- 

 stones would indicate that there would also be considerable difference in 

 the soft parts of the animal, from those with shorter loops ; and it then 

 becomes a question as to what changes these parts of a species can 

 undergo. The external form of those from this position is not very 

 unhke those from the Niagara group of Lockport, N. Y., except in gene- 

 rally being more ventricose ; that one seldom becoming gibbose. From 

 those occurring at Waldron, Ind., it varies in the finer striae, and in the 

 inequality of the valves. The Upper Helderberg specimens from the Falls 

 of Ohio scarcely vary externally from those of the shales of the Hamil- 

 ton group of New York, except in the coarser strise, but very materially 

 in the spires. 



I have not been able to examdne any European specimens of A. reticu- 

 laris; nor have I found any American examples of A. aspera in a condition 

 to show their internal characters, unless the A. spinosa of Hall can be 

 considered as of that species, which is scarcely probable. 



[Note. — The foregoing article was prepared for the Nineteenth Eeport 

 on the State Cabinet, as communicated to the Legislature April 2d, 1866; 

 but owing to the delay in printing, and the amount of material to be 

 set up before it, it has been detained until the present date. During the 

 autumn I have visited several localities of the Hamilton group in Northern 

 Iowa; at one of which (Waverly) I saw numbers of specimens of Atrypa 

 showing sections of the spires, and I observed that the volutions composing the 

 cones vary in number with the increase in age and size of the shell; while 

 specimens of the same size have about the same number of volutions. I might 

 also mention in this connection, that Prof. Hall has in his possession a small 

 specimen of A. nodostriala, sent to him by Dr. Rominger, which had been cut 

 to show the spires; but not far enough to reveal the loop; but by cutting a 

 little more, I found the loop well presei'ved. It is comparatively strong, and 

 has considerable forward curvature. The spires have only six volutions, owing 

 probably to the immature condition of the shell.] 



