GLYPHIOCERAS (BEYRICHOCERAS) SUBTRUNCATUM. 169 



faint lines of growth which on leaving the umbilicus curve forwards, then a little 

 backwards, and again more prominently forwards so as to form a crest just below 

 the periphery. One or two shallow constrictions are seen upon the east, hut the 

 test when present quite conceals them. 



Dimensions. 



Specimen in the Museum of 

 Science and Art, Dublin 

 (Geological Survey ('nil.). 

 Nantenan. 



Diameter of shell . . . .70 mm. 



,, umbilicus (edge to edge) . 7 ,, 



Height of outer whorl . . . . 39 „ 



,, above preceding whorl (about) . . 19 ,, 



Thickness of shell at umbilical margin . . 30 ,, 



By comparing these measurements, taken from an undistorted specimen, with 

 those of Glyphioceras (Beyrichoceras) truncatum (p. 107), the greater thickness of 

 the shell in the present species becomes apparent. 



Affinities. — On comparing the figures of this species with those of Glyphioceras 

 {Beyrichoceras) truncatum, the chief resemblance between the two will be clearly seen. 

 Both of them have the peripheral flattening more or less strongly emphasised in 

 the adult shell, and there is a similarity in the umbilical characters, smoothness of 

 the test, etc. ; but in the present species the shell is much thicker and more inflated, 

 and it is upon this character that the main distinction between the two species is 

 based. The peripheral lobe of the suture-line is also both broader and deeper in 

 G. (//.) subtruncatum than it is in G. (/?.) truncatum, its greater breadth naturally 

 corresponding with the superior breadth of its peripheral area. G. (/?.) inconstans, 

 de Kon., 1 is another species which will very well bear comparison with G. (II.) subtrun- 

 catum, and de Koninck points out its resemblance also to G. (II.) truncatum. It 

 differs from both in the greater size of its umbilicus in the adult stage of growth 

 and in the regular constrictions which occur in the young shell (De Koninck, 

 loc. cit., pi. xlviii, fig. 1). The larger size and angular margin of the umbilicus 

 in G. inconstans are the most striking differences between this species and G. 

 subtruncatum. 



The present species differs from G. obtusum in its more slender habit, in 

 the truncation or less broadly rounded character of the periphery, and in the 

 structure of the suture-line. 



Remarks. — This species lias so Ear been found chiefly in the south and south- 

 west of Ireland, in the counties of Tipperary and Limerick. 



1 Faune Calc. Carb. Belgique (Ann. Mus. Roy. d'Hist. Nat. Belgique, scr. Paldont., torn. v),pt. 2, 

 p. 120, pi. xlviii, figs. 4-9. 



