PLANETOCERAS GLOHATUM. 97 



test is in some places preserved in a state of perfection rarely seen in fossils, there 

 are exceedingly delicate longitudinal, apparently incised lines crossing the lines of 

 growth (PI. XIX, fig. 5). 



Dimensions. 



Diameter of shell (exclusive of 



free portion) 

 Height of outer whorl . 

 Width of umbilicus 



Affinities. — As I am acquainted with actual specimens of onlj"^ one species — the 

 one just described — I can say nothing vmder this head from personal knowledge. 

 Professor Hyatt ' describes a new species under the name of Planetoceras 

 retardaiii.m, which he compares with P. glohatnm. He says : " This (P. retardatum) 

 has the same outline to the aperture as in globatum, and a similar living chamber, 

 but is much smaller, and the whorls are not so stout or broad in proportion. The 

 ephebic (adult or mature) stage has, however, been fully attained, as is shown by 

 the rounded lateral zones of the whorls and the uncoiled character of the outer 

 part of the living chamber." 



I think I was in error in assigning Nautilus atlantoideus, de Kouinck, to the 

 present species,^ and 1 accept Professor Hyatt's correction when he says that N. 

 ailantoideus " difPers too much in the development of the 3'oung, if figured cor- 

 rectly by de Koninck, to be considered identical with P. globatum." ' 



BemarJcs. — The remarkable laterally expanded and contracted form of the free 

 portion of the body-chamber in this species differentiates it from all other Nauti- 

 loid shells of the Carboniferous rocks, the immature as well as the adult shell 

 exhibiting these features in an almost equally marked degree. 



This is a very common shell at St. Doulagh's, scarcely any series of fossils 

 from the quarries there being without s^Decimens of it. 



Localities. — St. Doulagh's, county of Dublin; " Kildare " (probably Clane). 

 Doneraile, county of Cork {fide Sir R. Griffith, in his list of localities apjiended to 

 M'Coy's ' Synopsis,' 1862 issue). 



' " Carboniferous Cephalopoda. " Seeoud paper. ' Geological Survey of Texas, Fourth Annual 

 Report,' 1892, p. 421. 



^ 'Cat. Fobs. Ceph. British Museum,' pt. 2, 1891, p. 128. 

 3 Loc. cit., p. 422. 



15 



