32 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



margins of the peripheral area are produced into a prominent rounded ridge, 

 outside of whicli and near to it there is a conspicuous keel extending the whole 

 length of the shell. In the young shell there are about ten similar keels, five on 

 each side, in the space between the ridge bounding the peripheral area and the 

 dorsal keel. In well-preserved young specimens these lines may be so strong as 

 to give a fluted appearance to the shell, but in the adult they become gradually 

 obsolete. The shield-shaped form of the section may originate in the earliest 

 stages of growth ; it is, at any rate, fully developed in a young specimen to which 

 I have had access, in which the diameter of the broken apical extremity is 6-5 mm. 

 In addition to the ornaments above described, the whole of the peripheral area 

 is covered with exceedingly fine, close-set, longitudinal lines, which persist in all 

 stages of growth, becoming relatively coarser in the adult shell ; they are not 

 present on the sides of the shell. Fine transverse lines of growth pass in a 

 sigmoid curve across the sides of the shell, continuing their course over the dorsal 

 keel, and forming upon the peripheral area a well-marked, backwardly directed, 

 hypouomic sinus, very conspicuous upon the body-chamber of old shells. 



The septa are numerous; in the young shell they are 3 mm. apart upon the 

 ventral area where its diameter is 18 mm. In a ver}^ large specimen, the dimen- 

 sions of which are given below, the septa are 8 mm. apart, where the diameter of 

 the shell measured upon its sides, from the keel bounding the peripheral margin to 

 the one in the median line of the dorsal area, is 45 mm. The chambers are thus 

 shallow. The necks of the septa are short and but slightly curved backwards. 



The siphuncle is cylindrical, and is nearly 3 mm. in diameter where the lateral 

 diameter of the shell is 22 mm.; thus it is aboat one-seventh the veutro-dorsal 

 diameter of the shell. 



Size. — The largest specimen known to me (a fragment contained in the 

 " Grainger Collection " in the Free Public Librarj^ and Museum, Belfast) consists 

 of the greater pai-t of the body-chamber and one air-chamber. Its dimensions 

 are as follows : — Greatest length, measured along the outer curvature, 195 mm. ; 

 dorso-ventral diameter of the smaller extremity 37 mm., of the larger 70 mm. 



Remarks. — At present only two species of Trlgouoceras have been recognised, 

 viz. T. paradoxicnm, J. de C. Sowerby, and T. aiijoceras, G. zu Minister,^ but there 

 appears to be some i-eason for believing that there is a third species. Professor 

 Hyatt remarks (' Geological Survey of Texas, Fourth Annual Report,' 1892, p. 405) 

 that Sowerby's species is quite distinct from the Belgian one, the latter being 

 apparently without any longitudinal ridges; the unique specimen described and 

 figured by de Koninck^ is, however, a cast, and would hence fiiil to show any 



1 Ci/rtocera aigokeros, ' Beitriige zur Petrefacteukuude ' (1st edit.), Is38, Heft 1, p. 33; 2nd 

 edit,, 1843, Heft 1, p. 5G, Taf. 1, figs. 7 «, 6 ; Taf. 2, fig. 1. 

 ^ ' Calc. Carb.,' j)t. 2. p. 7, j)l. xxxii, figs. 3, 3 «, 3 i. 



