<J4 CARBONIFEROUS CEPHALOPODA OF IRELAND. 



The apical part of the initial whorl, exposed in a young individual by breaking- 

 away the enveloping whorl, has a very slight curvature, the succeeding volution 

 completely covering its peripheral area. It is depressed dorso-ventrally like the 

 adult shell, which it resembles in miniature ; it expands rapidly ; the apex is scarcely 

 1 mm. in its lateral diameter; at a distance of 7 mm. this has increased to 3*5 mm. 

 The lateral areas are circumscribed, as in the adult shell, by two ridges which are 

 luuch more prominent than the others ; the dorsal area is evenly and gently 

 rounded, its surface is ornamented with nine or ten fine but distinct ridges, which 

 are crossed by minute lines of gi'owth. The ridges do not cover the extreme 

 apex, which is apparently smooth (PI. XXV, fig. 5). They increase rapidly in 

 strength and prominence as they advance from the apex. 



The septa in this species are very numerous ; the sutures are 5'5 mm. apart 

 upon the periphery, where the width of the latter is 30 mm. Their course 

 conforms in its direction to the shape of the periphery; upon the two elevations 

 they curve gently forwards and form a backwardly directed, shallow sinus in 

 crossing the median furrow. Upon the narrow lateral zones they curve slightly 

 backwards, taking the same direction on the umbilical declivities. 



The siphuncle is a little below the centre of the septum. 



The ornamentation, remarkably well developed in this species, consists of 

 strong, prominent, rounded, longitudinal ridges or keels covering the peripheral 

 area and bordering the lateral zones, but obsolete upon the umbilical declivities. 

 Five of these ridges occupy the elevated areas on each side of the median furrow 

 on the periphery ; the outermost of them forms the upper boundary of the very 

 narrow lateral zone, the lower boundary of which is constituted by the ridge 

 which forms the edge of the umbilicus. A ridge, much less prominent than any 

 of the others, occurs near the summit of the umbilical declivity just beneath the 

 keel bordering the latter. This supplementary I'idge is present in all stages of 

 the growth of the shell, beginning with the initial innermost whorl. Though 

 resulting from a vertical thickening of the test, the ridges nevertheless leave 

 distinct elevations upon the cast. They are divided upon the periphery by spaces 

 of about their own width, but a slightly greater space separates the outermost 

 keel from the one bordering the umbilicus, this space being what I have called 

 the lateral zone, whose plane is at right angles to that of the periphery. The 



.nny that liave yet beea described, as has proved to be the case with Vestinantilus cariniferus (see 

 above, p. 84), Signs of the adult stage are generally to be discerned in some modification of the orna- 

 ments of the test, which on the one hand may become coarser, or on the other obsolete or nearly so ; the 

 latter condition usually indicating the senile, or, to use Professor Hyatt's expression in its broader 

 sense, gerontic stage. It must be observed that these signs of maturity are not present in the indi- 

 vidual whose diameter I have given above, which is, moreover, broken anteriorly ; hence the query. 



