BRAXCOCERAS ORXATISSDIU.M. 133 



Description. — Shell cliscoklal, compressed, with closed umbilicus; greatest 

 thickness at about ouc-fourth of the height of the whorl from the umbilicus, about 

 seven-tenths of the diameter of the shell; height of outer whorl about three-tihlis 

 of the diameter of the shell. Number of whorls unknown ; inclusion comi)lete. 

 Sides very slightly convex, merging in the narrowly rounded and somewhat 

 truncated periphery. Umbilicus closed with a callus, which extends to the centre 

 of a shalloAv funnel-shaped depression. Whorls subcordate in cross-section. 

 Body-chamber unknown, the septate portion alone being present in the sole 

 individual found. 



The septation is clearly exhibited, not only by the removal of the test, but also, 

 owing to a fortunate transverse fracture, the greater part of a septum is exposed, 

 thus giving an interior view, which shows the digitate form of the lobes and th e 

 cavities left between them (PI. XXXVII, fig. 1 b). 



The ornamentation is well marked and very elegant; it consists of distinct 

 raised lines or fine ribs, separated by spaces which are a little wider than the 

 latter. The ribs are from about '75 to 1 mm. apart. Proceeding from the 

 umbilicus they describe a wide-spread, forwardly directed curve, which merges in 

 a greatly narrowed and backwardl}- directed sinus upon the peripher}^ indicating 

 a deeply indented hyponomic sinus. The siphuncle is seen in section in the centre 

 of the peripheral lobe, Avhere it has a diameter of 2 mm. 



Dimensions. 



Type specimen in 



Museum of Science and 



Art, Dublin. 



Diameter of shell . . . . (JS mm. 



Height of outer whorl (dorso-ventral) . . 41"5 „ 



„ above preceding whorl . . .23 ,, 



Thickness at umbilical margin . . . 4() ,, 



Afinities. — Among the species belonging to this genus hitherto described there 

 is none that can be strictly compared with the present one. 



The Belgian Carboniferous species, Brancocems rotatoriiini, de Kon.,^ sp., 

 resembles the latter in its rather depressed form and closed umbilicus, but the 

 sutures differ in a marked degree. As de Koninck's species is only represented by 

 an internal cast, no comparison can be made of the ornaments of the two species. 



Remarks. — This beautiful form, the only one known to occur in the British 

 Isles, is represented by the specimen which is the subject of the above description. 

 Its true affinities had not been recognised until Mr. Crick, in looking over the small 

 collection of Goniatites in the Geological Survey Collection (Dublin ]\ruseum of 



1 '• Fauue Calc. Carb. Belg.," 1880 (' Ann. Mus. Roy. d'Hist. Nat. Belg.,' toni. ii), pf . ii, p. 94, 

 pi. xlvii, fig. 12. 



