450 GEOLOGY OF THE BLACK HILLS. 



CEifAS Conrjid {lliuiiuhifi atniitlifer Mort.). We have in tliis collection (juite 

 51 miinl)or of individual.s of the species herein described under the name P. 

 meeJcamoif, several of wliicli show the prolonged curved portion of the 

 smaller linil), and two individvals of I\ crassiim ; but all the evidence fur- 

 nished b\- these s})ecimens is of a character opposed to either of these 

 suppositions. The peculiar bending of the smaller limb of the first of these 

 species would seem to ])reclude the })robability of the larger part of the 

 shell followiufj the (uirvature of the earlier and returninw- over its extremitv 

 so as to infold it; while the assum])tion of old age charactei's in the contrac- 

 tion of the distance between the rings, as seen on several of them near 

 the end of the tube, would seem to indicate that neither of these theories 

 is con-ect; but that the shell is essentially complete at about this stage of 

 gi'owth. Beyond this, among all the individuals of these two species 

 observed, none of them are septate beyond the commencement of the 

 fold, and at this point the septa are more crowded than below. We are 

 also much inclined to believe that the folding of the siphonal surface of a 

 cephalopod upon itself, after having coiled in the opposite direction, is a 

 feature seldom observed. 



Although several of the examples of P. meekannm, before us, show 

 the bending of the smaller limb, as seen in the example figured, none of 

 them are perfect at the smaller extrernity ; so we are still uncertain as to 

 its character or form during the earlier stages of its existence. In the rock, 

 forming the matrix of some of these specimens, we have noticed fragments 

 of a smell Ammonite-like shell, having the same form of section and sur- 

 face markings with the smaller parts of these, and we have been induced 

 to infer that possibly they may be the earlier portions of the same shell, 

 and that possibly, in its younger stages, it ma}^ have been minutely pla- 

 norbifonn; but the fragments are two imperfect to determine the question. 



We have also noticed another feature on the larger individual of P. 

 crassuni (Plate 1(5, Fig. 3), which we had at first supposed to be the result 

 of accident; but which on further examination we find to be a thickening 

 of the external surface of the shell, and which does not in any way inter- 

 fere with or affect the septa or the internal portion of the shell. It com- 



