(No. 4124). 









Clane. 



112 mm. 









90 mm 



20 „ 









17 „ 



15 „ 









13 „ 



55 „ 









40 „ 



29 „ 







iboi 



it) 



20 „ 



39 „ 





,, 





30 „ 



GLYPIIIOCERAS (BEYR1CH0CERAS) TRUNCATUM. 107 



tion, casts sometimes exhibiting- them. The shell is for the most part, however, 

 quite smooth. 



Dimensions. 



Specimens in the Museum of Science and Art, Dublin. 

 Geol. Surv. Coll. Lisnakerry General Coll. 



Diameter of shell 



,, umbilicus (edge to edge) 



,, ,, (suture to suture 



Height of outer whorl 



,, above preceding whorl (about) 

 Thickness of shell at umbilical margin 



These dimensions bring out very clearly the flatness of the shell in this species, 

 its most conspicuous feature, and the one by which it is most easily recognised. 



Affinities. — It is quite evident that this species is closely connected with the one 

 described under the name of Glyphioceras (Beyrichoceras) subtruncatum (p. 108). 

 It differs from this in being flatter in form and in the possession of a smaller umbili- 

 cus ; the truncation of the periphery appears to be equally strongly marked in the 

 two species, and to vary in degree in different individuals. The surface markings 

 are the same in both species, the distinctness of the transverse lines depending very 

 much upon the state of preservation of the shell. Under favourable conditions they 

 constitute in the present species a fairly distinct ornamentation. The constrictions 

 do not in any case constitute a remarkable feature, except in some very young 

 individuals ; they are only seen upon the cast, as the test completely covers them. 

 Taking the character of the suture-line into account, it appears that the chambers 

 are somewhat shallower in G. (IJ.) truncatum than they are in 67. (//.) subtruncatum. 



Remarks. — The brief description of this species given by Phillips runs as 

 follows: — "Very depressed, back (in adults) truncate; umbilicus open; fine trans- 

 verse bent striae. " The type specimen being preserved (" Gilbcrtson Collection," 

 British Museum), the identity of the species was secured, though the feature upon 

 which the author founded it, the truncation of the periphery, is not always so con- 

 spicuous as it might lie judged to be according to Phillips's figure ('Geol. Yorks.,' 

 pi. xxix, fig. 21). Among the specimens before me the distinctness of the peri- 

 pheral flattening varies in different individuals, and while in some it is quite 

 apparent in others it is scarcely discernible This variability, however, dors not 

 deprive the species of its individuality, its compressed form, as already stated, 

 supplying the principal specific feature. 



Glyphioceras (Beyrichoceras) truncatum is one of the most common and most 

 widely distributed of the Carboniferous Goniatites in Ireland ; it varies considerably 

 in size, and to some extent in thickness, some individuals being much more com- 



