76 CARBOiNIFEROUS CEPHALOPODA OF IRELAND. 



As the shell grows towards the adult stage the median elevation becomes 

 depressed and indistinct and gradually almost obsolete, the strong ridges on each 

 side of it disappearing entirely in the senile stage, the body-chamber being here 

 quite smooth. The umbilical declivities— for in this species there are no " sides," 

 strictly speaking— are gently curved, becoming somewhat steeper when the body- 

 chamber is reached. The zoue of impression, which can only be seen on the 

 body-chamber of one of the specimens before me, is so shallow as to be scarcely 

 perceptible. It must be more pronounced in the young shell in which a median 

 peripheral elevation is developed, as already described. 



The septa are not seen, the only indication of their form, and that an 

 imperfect one, being the impression left by the suture-line at the base of the 

 body-chamber. Here, occupying the centre of the periphery, their outline 

 describes a broad and shallow sinus, whose convexity is directed towards the 

 aperture ; this sinus is bounded on each side by an acute, posteriorly pointed 

 lobe, the outhne of which is continued obliquely upwards and outwards till it 

 reaches the peripheral margin, at which point it bends backwards in a brief curve 

 upon the extremely narrow zoue representing the sides of the shell. The 

 curvatures and lobes here desci'ibed are coincident with and originate from the 

 elevations and furrows developed upon the periphery. 



The siphuncle is situated apparently a little above the centre. It is very 

 obscurely seen, owing to the spaces between tlie septa being tilled up with calc- 

 spar. 



The ornamentation consists of longitudinal ridges of variable strength. Upon 

 the dorsal or anti-peripheral area of the adult shell there are none; my material 

 does not enable me to say whether the young shell had any upon this part. 

 There are four upon the umbilical declivity, the two lower oeing less pro- 

 minent than the upper ones ; the space between the lowest ridge and the one 

 immediately above it is slightly wider than that between the other ridges. A 

 very strong and prominent ridge forms the edge of the umbilicus, and at the same 

 time that of the periphery ; this is acute in the young shall, but it becomes 

 rounded as the shell attains maturity, till finally in the senile stage it is obsolete, 

 its position being mai'ked only by a slight elevation. 



The periphery in the young shell has eight ridges, excluding those forming its 

 border; three of them are on each side of the median elevation, the inner ones 

 forming the boundary of the furrows; the latter are shallow; the median elevation 

 is not quite as high as the lateral ones; down its centre are two ridges which are 

 much less prominent than those on each side of it, and as the shell grows they 

 gradually become obsolete at about the end of the first wliorl. The lateral ridges 

 of the periphery appear to extend as far as the body-chamber, but to what length 

 they go beyond this I am unable to say, as the test is not preserved upon 



