2 PROCEEDINGS OF THE NATIONAL MUSEUM vol.76 



In order to make clear the following description it should be ex- 

 plained that Mitrosplra is assumed to have normal dextral coiling. 

 Mitrosfira seems in fact to be an everted MacluHtes^ the flat lower 

 surface of that genus being produced into a spire. It is tentatively 

 held, therefore, that the spire of Mitrospira is the lower and the 

 umbilicated side the upper surface of the shell. 



The genus as known is characterized by individuals of large size. 

 The lower side of the shell is produced into a fairly high spire, 

 while the upper side shows a wide umbilicus. The relative height 

 of the spire is variable, due to greater or less overlap of the whorls 

 on those preceding. The umbilicus is wide and open, the tubular 

 cavity extending to the apex of the spire. The upper free margin 

 of the whorls next the umbilicus is subangular. As shown by the 

 growth lines and a fairly perfect aperture, this keel marks the apex 

 of a deep reentrant notch. The keel is then considered the probable 

 equivalent of the notch keel of the typical Pleurotomarids. An in- 

 teresting feature of the genus is a progressive filling of the living 

 chamber by secondary deposits of lime. This is noted in the descrip- 

 tion of the species and shows clearly in the section figured. The 

 deposit is closely comparable with the secondary filling of the older 

 camerae of certain cephalopods. 



It is here considered probable that Mitrospira is a direct deriva- 

 tive of Machi/rites, having originated as an abrupt saltation. Both 

 in the preceding portion of the Pogonip, of approximately Beek- 

 mantown age, and the Chazyan portion Maclurites and its congeners 

 are abundant not only in numbers but also in diversity of specific 

 and possibly generic types. No antecedent form to Mitrospira has 

 been found, which, though negative evidence, is suggestive. On 

 the other hand, the considerable range in height of spire to be seen 

 in Mitrospira^ coupled with its essential identity of structure with 

 Maclurites, indicates a close genetic relationship with the latter 

 genus. A tendency toward the formation of a spire is shown, so 

 far as known, only on this horizon, except as noted later in the very 

 late Ordovician. In the Pogonip one species of Maclurites at times 

 shows a very slight eversion of the whorls, giving a slightly convex 

 outline to the lower surface. It is of interest to note that otherwise 

 no such tendency toward spire formation has been seen on examina- 

 tion of large numbers of Maclurites from the Middle and Upper 

 Ordovician, except in the genus Palliseria. 



The only other known gasteropod with which Mitrospira may be 

 compared is Palliseria Wilson.^ This genus is compared with Maclu- 



^ Wilson, Alice E., A new genus and a new species of gasteropod from the Upper Ordo- 

 Tician of British Columbia : Canadian Field-Naturalist, vol. 38, No. 8, pp. 150-151, October, 

 1924. 



