GASTRIOCERA.S CIRCUMPLICATILE. 201 



what broad sinus upon the periphery. Constrictions to the number of two or three 

 to a whorl are present; these are rather inconspicuous upon the test, but make 

 deep grooves in the cast, their course following that of the transverse lines. 



Di 



IHt'HSlOltS. 



Specimen from the "Cliffs of Moher" 

 in the Dublin Mus. Sci. and Art 

 (Gool. Sui-v. Coll.) (No. 1801 K). 



Diameter of shell .... 22'0 mm. 



,, umbilicus . . . 8*5 ,, 



Height of outer whorl . . . 10 # ,, 



„ above preceding whorl . . . 7'0 ,, 



. 11-0 „ 



Affinities. — While there can be no doubt that a somewhat close relationship 

 exists between the present species and G. Marianum, de Venn, the distinctness of 

 the two species is made manifest by contrasting their proportions. These may be 

 best realised if thrown into a tabular form : 



Gastrioceras circumplicatile. Gastrioceras Marianum. 



Umbilicus about two-fifths of the diameter of Umbilicus about three-fifths of the diameter of 



the shell in width. the shell in width. 



Breadth and height of whorl nearly equal. Breadth of whorl about two and a half times 



its height. 



Thickness of the shell about one-half of its Thickness of the shell about two-thirds of it* 



diameter. diameter. 



Height of outer whorl not quite one-half of the Height of outer whorl about one-third of the 



diameter of the shell. diameter of the shell. 



It is in the ornamentation of the shell that a striking similarity between the two 

 speeies is noticeable, the most prominent feature being the elongated tubercles or 

 plications decorating the margin of the umbilicus, from which arise the series of 

 beautiful curved lines crossing the sides and peripheral area of the shell. These 

 ornaments are common to both species, and give them ;i very distinctive character. 



Except in the great width of the umbilicus and in its having marginal orna- 

 ments, there is no special resemblance between the present species and G. Listeri, 

 nor between the latter and 67. Marianum, as urged by de Verneuil. 1 



Karpinsky - compares 67. Marianum with 67. Jossae, pointing out the more 

 prominent character of the spiral ribs in the latter, and the greater width of the 

 umbilicus in G. Marianum. But the marginal ornaments in (!. Jossae are much more 

 prominent both in the young and in the adult than they are in <!. Marianum and 

 G. circumplicatile ; they also early develop into strong tubercles. Neither in G. 



1 'Geologic Russie d'Europe,' vol. ii, Paleont., L845, p. 369, pi. x.wii, figs. 2a — c. 



'-' 'Mem. Acad. Imp. Sci. St. Petersbourg ' (7), vol. xxxvii, 1889, p. 49, pi. iv, figs. 2 a — c. 



