NATURAL SCIENCES OP PHILADELPHIA. 261 



remains to determine what are its nearest relations. Its simple- 

 edged septa confine it to the group, of which the modern Nautilus 

 may be taken as a t} T pe. But the complex nature of its septa has 

 no analogy among the modern genera, even geologically speaking. 

 AVe must look for its relations among the palaeozoic forms, such as 

 Endoceras, Aciinoceras, and more especially Beatricea. In fact, 

 it seems in some respects to be intermediate between the last, as 

 described by Hyatt 1 and the two former ; and again to connect 

 both groups with the Orthoceridse proper. In Orthoceras we have 

 direct communication from one chamber to the next. In Endo- 

 ceras, Actinoceras, and the allied genera, the position of the siphon 

 is occupied by a shelly tube which seems to have allowed no such 

 communication beyond its own walls. In Beatricea the central 

 column is described as consisting of a chain of small hollow 

 chambers, not continuous either with each other or with the en- 

 circling chambers. In Polorthus, unlike Orthoceras, the tubes 

 are not membranous, but shelly. They can be compared in their 

 enveloping character to the column of Beatricea, of which Hyatt 

 says, "the central chambers are imperforate, generally deeply 

 concave, and set upon one another like a pile of Chinese teacups." 

 By elongating these "teacups" into a series of laterally com- 

 pressed cones, we have the column of Polorthus, with the differ- 

 ence that in the latter each cone is soldered to, or more properly, 

 is a continuation of one of the outer septa. The nature of the 

 matrix is such that I have not been able to demonstrate, bej^ond 

 a doubt, the character of the apex of these cones, but I believe it 

 to have been perforated by a minute slit ; a fact which would con- 

 nect Beatricea with Endoceras. On the other hand, while the 

 base of each cone, where it leaves the transverse septum, is appa- 

 rently closed completely by the next cone fitting tightly into it; 

 there is at least a rudimentary, if not a real connection between 

 the outer chamber and the interior of the column, thus showing a 

 nearer connection between Polorthus and Orthoceras than between 

 the latter and the other two quoted genera. 



The muscular scar visible on the casts of the interior just above 

 the last of the septa is not without its analogy. I have found 

 such impressions in all the modern species of Nautilus, but more 

 especially in N. jjompilius, 2 where it consists of two broad scars 



1 Amer. Jour. Sci., 1865, p. 261. 



2 See Waagen, Ueber die Ansatzstelle der Haftniuskeln beim Nautilus 



