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reported as found at Colorado City Mineral Springs, and is in 

 Museum of Comparative Zoology. 



Helicoceras umbilicaium, Meek, Invertebrate Paleontology, PL 

 xxii, Fig. 5, is probably a close ally of Stevensoni, but my informa- 

 tion is not satisfactory. 



Heteroceras Conradi, as figured by Whiteaves in Mesozoic Fossils, 

 i, Pt. ii, PL xii, and supposed to be identical with Amnionoceratites 

 Conradi of Morton is also a form that is not sufficiently well-known 

 to be referred to its proper genus. The costae have no tubercles 

 and resemble those of the young of Nostoceras Stantoni and helicinum. 

 There is also obviously a retro versal gerontic volution shown by 

 Whiteaves in his Fig. 3, and there is apparently no contact furrow, 

 the mode of growth being heiicoceran and not turrilitean. 



It may be useful in connection with these descriptions of phylo- 

 gerontic forms to note the fact that there are some series of true 

 ancyloceran forms in this country having shells revolving in sym- 

 metrical spirals in the same plane, and not having helicoidal ephebic 

 stages. They are similar to most of the European series, and it is 

 not advisable to name them till proper comparisons can be made. 



Ancyloceras percostainni and Renwndi, Gabb, described in 

 Paleontology of California, are good examples of species having this 

 kind of spiral. 



Lindigia. 



This genus Karstens,* with Lindigia helicoceroides as the type, has 

 (in this small species)- linear, untuberculated costse, a heiicoceran 

 spiral in the ephebic stage, and comparatively a very large and long 

 retroversal gerontic volution. 



Heteroceras Conradi of Whiteaves may be a species of this genus 

 occurring in North America, but Lindigia has peculiar ventral 

 crests in the costae of the anagerontic substage that are quite dis- 

 tinct from those figured by Whiteaves in his species. 



Helicoceras si7nplicostatum of Whitfield resembles this species, but 

 it may nevertheless be the parephebic substage of some other tuber- 

 culated species which has lost its ephebic tubercles. 



Nostoceras, f 11. g. 

 The species of this genus have a close-coiled unsymmetrical shell 

 during the ephebic stage and are true turrilites. There are two 



* Geo!, dc la Culombic, Venezuela, etc., PI. i, 1S56. 

 1 1X6 (TTO?, a return. 



