(lASTRIOCERAS CIKCUMI'LICATILK. 201 



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

 to a whorl are pi'esent; these are rather inconspicuous upon the test, but make 

 deep gi-ooves in the cast, their course following that of the transverse lines. 



iJiinciiiilimS. 



SpeciiiH'ii frmii the " Cliffs of Mohi-r " 

 in tho Dublin Mus. Sci. ami Art 

 (Guol. Sm-v. Cull.) (Nu. WOl K). 



Diameter of she'll .... 22-0 mm. 



,, umbilicus . . . 8"-5 ,, 



Heiglit of outer whorl . . . lO'O ,, 



,, above preceding whorl . . . 7'0 „ 



Thickness at umbilical margin . . . 11"0 ,, 



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

 exists between the present sjiccies and (J. Muriaiuim, de Vern., the distinctness of 

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

 best i-ealised if thrown into a tabular form : 



Gastrioceras circmtqilicatile. Gastrioccnts Mariaiiiini. 



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



the shell in width. the shell iu width. 



Breadth aud height of whorl nearly equal. Breadtli 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 its 



diameter. diameter. 



Height ^)f outer whorl not i|uite one-half of the Height of outer whorl aliout oue-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 

 species is noticeaijle, the most prominent feature being tlie elongated tubercles or 

 plications decorating the margin of the umbiHeus, from which ari.si' 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 a very distinctive character. 



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

 nients, there is no special resemblance between the present species and 6'. LUlcri, 

 nor between the latter and (J. Mariauuni, as urged by de Verneuil.' 



Karpiiisky" compares G. Murbinuiii with (r. Junsse, pointing out the more 

 prominent character of the s[)iral ribs in the latter, and the greater width of the 

 uml)ilicus in '/. }[iiri<iiiinii. But the maiginal ornaments in (1. Johsx -MX' iinieli more 

 pi'ominent Ijoth in tlie young and in the adult than they are in (!. Mdiinnniit and 

 (f.rirrampiicdfilr; tliey also early develop into strong tubercles. Neither in (1. 



' 'Gi'ologie Russie d'Eurojie,' vol. ii, I'ah'niiL, 1845, ]>. oO'.*, \>\. x.wii, figs. 2 a — c. 



- 'Mi'm. Acad. Imp. Sci. St. Pi'tersbourg ' (7), vol. xx.wii, IHS'J, p. 4'.', pi. iv, figs. 2 a — c. 



