35 



The sculpture consists of transverse raised ribs, which although acute 

 and pi'omineiit, are always narrower than the deeply concave and 

 rounded grooves which separate them. "What may be conveniently 

 termed the primarj- ribs, commence at the sutures, bifurcate about the 

 middle of the sides, pass over the periphery, and reunite at a corres- 

 ponding point on the other plane of the shell. From thirty-five to 

 thirty-six of these primary costse can be counted on the last volution. 

 At the point where they begin to bifurcate, they swell up into narrow 

 and transversely elongated elevations, which can scarcel}^ be termed 

 tubercles, at least if that term is used solely to convey the idea of 

 circular prominences. Sometimes, especially near the apertnre, the 

 bifurcation of the primaries is obscm-e, and the appearance then presented 

 is that of pairs of continuous ribs, separated by one or two shorter ones. 

 Occasionally, also, two bifurcating and primary costffi immediately 

 succeed each other, without the intervention of an}^ secondary rib. The 

 secondary costal are invariably simjjle, as well as being always shorter 

 than the primaries. They encircle the outer half of the whorls only, 

 and do not extend to the sutures, but become obsolete and disappear 

 about the middle of the sides, just where the primaj'ies begin to bifurcate. 

 In the last whorl but one the arrangement of the costaa is very uniform ; 

 a single bifurcating rib almost invariably alternating with a shorter, 

 simple one; but near the ajoerture the grouping is much more variable 

 and irregular. As seen along the peripheiy, the whole of the ribs are 

 both equidistant and of the same height ; they are not separated by a 

 narrow space which encircles the siphonal edge, (as is the case in some 

 nearly allied species,) but pass uninterruptedl}^ over it. 

 Septation unknown . 



The greatest diameter of the largest known specimen, (to which the 

 above description refers exclusively, and which must be regarded as the 

 type of the species), is seven inches and two lines; the width of the um- 

 bilicus, from suture to suture, is about three inche and eight lines. 



Besides the specimen just described, there are six small Ammonites 

 which most probabh' belong to the same species, although the interme- 

 diate stages of growth have not yet been observed. The most perfect of 

 these, which measures about two inches and three lines aci'oss, will be 

 found represented at figure I. of Plate IX. The others are mostly mere 

 fragments, but they show that the sculpture is very variable in different 

 individuals of the same size, besides giving other information which hap- 

 pens not to be afforded by more perfect individuals. As compared with 

 the type, the fossil figured on Plate IX. has its outer whorl proportio- 



