148 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



the following species, it will be observed, group themselves very naturally together, 

 viz. Pcririjdm Foordi, P. suhplicdtilix, P. rotuUfonnis, and P. Bailyi. As might be 

 anticipated, the young or smaller shells in this group are difficult to discriminate, 

 and, unfortunately, a large gradational series of individuals, so much to be desired 

 among fossils, both on taxouomic and biological grounds, is here wanting. 80 far, 

 however, as the, material indicates, the two forms figured on PL XL seem to be 

 quite distinct, the main feature (one which does not appear so striking in the 

 figures as in reality) consisting in the more compressed shape of P. rottdiforDiis, as 

 compared with that of P. Jiaih/i. The ribbing also is seen to be more scanty in the 

 former than it is iu the latter. 



Locality. — St. Doidagh's, county of Dublin. 



Pebicyclus I!uti'i,ik(h;mis, ^'. C. Crirk. Plate XL, figs. 4—7. 



1890. Pekicyclus rotulifoemis, G. C. Crick. Ami. Mag. Nat. Hist., ser. 7, 



vol. iii, p. 434, fig. 3. 



Descrijjfioii. ^HheW discoidal, rather compressed, somewhat widely umbilicated; 

 greatest thickness at the umbilical margin, about three-eighths of the diameter of 

 the shell ; height of outer whorl in about the same ratio to that diameter. Whorls 

 not less than four and a half, but possibly exceeding that number, the absence of 

 the body-chamber in all the specimens that have come under my notice making an 

 exact computation unattainable. Inclusion two-thirds. Umbilicus of moderate 

 depth, about three-tenths of the diameter of the shell in width, exposing the inner 

 whorls ; the sides slightly sloping, not very deep, with suljaiigidar margins. Whorls 

 subcordate in cross-section, very slightly higher than wide, the difference betAveen 

 the two dimensions being scarcely appreciable, as will be seen in the table given 

 farther on. The indentation of the preceding whorl amounts to about one-third 

 of the height of the last one. Sides very slightly convex, merging in the narrowly 

 rounded periphery. LTmljilical zone (jmlging from natural casts only) fairly 

 well defined, sloping towards the centre of the umbilicus, thus making an obtuse 

 angle with the sides. ]3ody-chamber occupying rather more than an entire Avhorl. 

 The suture-line of this species (PI. XL, fig. 7) is here figured for the first time. 



Test (judging by the fragments of it preserved) ornamented with feeble ribbing, 

 which appears to be rather more conspicuous upon the cast. The ribs bend rather 

 abruptly backwards upon the sides of the shell, and make a rather narrow and 

 linguiform liy^iouomic sinus upon the periphery. Periodic constrictions occur at 

 iri-egular intervals and form very deep grooves upon the cast, but are shallower 

 where the test is present ; there are generally about seven of them in the circuit 

 of a whorl, and they form a very conspicuous feature iu this species. Their direc- 



