CCELONAUTILUS PLANOTERGATUS. 55 



inches (254 mm.). The largest I have seen (Museum of Science and Art, Dublin) 

 measures 160 mm. in its greatest diameter, the diameter of the umbilicus measured 

 from the opposite margins being 75 mm. The central vacuity in another specimen 

 measures 14 mm., thus far exceeding in the latter dimension de Koninck's figured 

 specimens. The periphery is 45 mm. in width, the sides about 50 mm, in the 

 widest part available for measurement. 



Affinities. — G. gradns, though resembling the present species in the markedly 

 quadrate form of its whorls, is easily distinguished therefrom by its much slower 

 rate of increase, the much greater width of the peripheral area compared with 

 the sides, and by its large central vacuity. Moreover it does not appear to 

 attain the large size to which C. jjlanofergatiis develops. 



BemarJiS. — Unfortunately the specimens available for my present study of the 

 species, which includes M'Coy's type, are distorted by rock pressure ; moreover 

 they do not exhibit the complete growth of the shell through all its stages, hence 

 my description is to some extent imperfect. 



On the whole, de Koninck's figures of G. planotergatns give me the impression 

 of a shell with less distinctly quadrate whorls and a much shallower umbilicus 

 than the forms which have come under my notice. 



This species was originally named Navtlhis hexagonus by de Koniuck,^ but, as 

 the name had already been used by J. Sowerby for a Jurassic species, it had to be 

 discarded, and the specific na,v[ie planotergatus^ given hj M'Coy " to what appeared 

 to be the same form, was accepted by de Koninck^ and subsequent writers. 

 Prof. Hyatt,* however, on comparing specimens of the Belgian species {Nautilus 

 hexagonus, L. G. de Koninck) contained in the Museum of Comparative Zoology, 

 Cambridge, Mass., with M'Coy's species, comes to the conclusion that they are 

 distinct. He remarks that the Belgian species " has a whorl in the young with 

 more convergent^ sides [than those of G. iilanotergatus], and is not so broad 

 proportionately on the abdomen [periphery], and has therefore the gerontic 

 [senile] form at an earlier stage." He thinks, therefore, that it is better to retain 

 the name of G. hexagonus ; on his authority I have accordingly omitted it from 

 the list of synonymy of G, pJanotergatns. 



Localities. — Cork (near the city) ; Rathkeale, county of Limerick. 



1 ' Descrip. Anim. Foss. Terr. Carb. Belgique,' 1841, pi. xxv, fig. 1. 



' ' Synop. Carb. Foss. Ireland,' 1844, p. 18, pi. ii, fig. 2. 



5 "Faune Calc. Carb. Belgique" ('Ann. Mus. Eoy. d'Hist. Nat. Belgique,' Paleout., torn, ii), 

 pt. 1, p. 117, pi. xxvi, figs. 1—3. 



* " Carboniferous Cephalopods." Second paper. ' Greological Survey of Texas, Fourth Annual 

 Report,' 1892, p. 408. 



5 This term signifies that the sides of the whorl are inclined outwardly or towards the peri- 

 phery, instead of towards the umbilical margin (" divergent "). 



