80 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



ill section, the peripheral area making a broad flattisli arch ; the sides, which 

 form the steeply descending umbilicus, diverge at an obtuse angle from the 

 margin of the latter ; they are somewhat inflated in the lower balf and slightly 

 concave in the upper, resembling those of Vest, cariniferus in this respect. The 

 contact or inclusion of the whorls is very slight, and, so far as can be made out 

 from a section, there is no perceptible zone of impression. Owing to this slight 

 amount of overlapping of the whorls, part of the peripheral area is exposed in 

 the umbilicus, the inner whorls thus making a deep channel where they abut 

 against those that embrace them. The umbilical border is marked by a strong, 

 rounded rim, which becomes very prominent in the adult and senile stages. 



In the young sliell the rim is about as prominent as it is in the adult of Vest, 

 cariniferus, J. de C. Sow., and other allied species. The conical apex of the 

 young shell is ornamented with a series of close-set, fine, longitudinal ridges, and 

 these are crossed by minute transverse strige, which, cutting the former, impart a 

 beautifully crenulated appearance to this part of the shell as seen under the lens. 

 After the first whorl is passed the spiral lines begin to widen, and those of the 

 lower half of the whorl entirely disappear, leaving five ridges, the two lower ones 

 being very faint, till finally they all become obsolete, leaving only the transverse 

 lines. These are regularly arranged and very numerous, their distance apart in 

 the adult shell being almost exactly 1 mm. From the appearance presented by 

 one of the specimens it would seem that they imbricate, but this is not very 

 distinctly seen. They form upon the peripheral area a very distinct and deep, 

 backwardly directed sinus, indicating the presence of the hyponomic sinus of the 

 aperture. On each side of the peripheral area there are three or four faint 

 ridges in the young shell, but before the second whorl has been reached they 

 have entirely disappeared. In a large individual in the senile stage of growth 

 obscure folds and tubercles are developed upon the peripheral area and in one or 

 two places upon its margin, but these are not sufticiently prominent to alter the 

 general features of the shell. 



The septation is not known, a cross-section of one of the specimens showing 

 that it has been destroyed internally by mineral action ; the presence of the test 

 prevents the sutures from being seen externally, supposing them to have been 

 preserved. 



The siphuncle is about twice its own diameter from the peripheral margin 

 (PI. XXII, fig. 5 c). 



The body-chamber is imperfect in the specimen in which it is seen (PI. XXII, 

 fig. 5 a). A very fine but distinct ridge runs along the median line of the 

 peripheral area upon the cast of the body-chamber ; no traces of this ridge are 

 seen upon the test. 



The test is rather thick, especially upon the umbilical border, where it forms 



