56 CARBONIFEROUS CKPHALOPODA OF IRELAND. 



b. CcELONAUTlLUS DooHYLENSis, sp. nov. Plate XIX, figs. S a, h, 4 a, b. 



Description. — Shell small (young ?), thick-discoid, rapidly increasing in dia- 

 meter, with probably three or three and a half whorls, all exposed in a very deep 

 umbilicus, the outer one with extremely steep, step-like sides. The section of the 

 shell is distinctly quadrate, but somewhat wider than high, and it is tetragonal. 

 The peripheral area is broad and very slightly concave in the centre ; the margins 

 are acute. The sides in the inner whorls are considerably narrower than the 

 peripheral area, but in the body-chamber the reverse is the case, the sides 

 increasing rapidly in diameter towai'ds the aperture. The umbilical declivity in 

 the body-chamber is very steep, and terminates at the edge of the peripheral area 

 of the whorl it embraces. 



The septa are not seen, but the form of the sutures can be made out from the 

 base of the body-chamber ; they are deeply concave on the sides of the shell, and 

 form rather a deep, backwardly directed sinus upon the peripheral area. 



The siphuncle is very small, and situated a Httle above the centre of the 

 septum. 



The ornaments consist of four, irregularly spaced, vei'y fine, thread-like, 

 longitudinal lines on each side of the pei'ipheral area; lines of growth are also 

 faintly discernible, crossing the peripheral area transversely. 



Dimensions. 



Diameter of shell 



Height of whorl (dorso- ventral) . 



AVidth of whorl 



,, umbilicus (from edge to edge) 

 „ ,, (from suture to suture) 



Thickness of whorl at umbilical edge 



Specimen froii 

 Katlikeale. 



50 mm. 



— ^ )5 



20 „ 



21 „ 

 19-5 „ 



Affinities. — It was at first thought that this shell might be the young of 

 C. planotergatus, but its much more rapid rate of increase, as evinced not only in 

 the diameter of the sides but in the remarkable contrast in size between the last 

 whorl and the one preceding it, entirely separates it from that species. I am not, 

 however, able to say positively whether the specimen figured is a young or an 

 adnlt one of its species, but the indications are in favour of its being the former; 

 that is, there is no rounding off of the lateral angles indicative of the adult stage 

 of growth, and its small size also suggests, though it does not of course 

 prove, its immaturity. 



