244 DinococJdea ingens froTTi the Wealden Beds, Hastings. 



To put the whole matter to the test, careful measurements were 

 taken of the portions known to belong to the dextral specimen 

 photographed by Mr. Pottinger. The result went to show that they 

 were in accord with a logarithmic spiral having the ratio of 1-02. 

 A diagram (Fig. 1) was then constructed on this basis, the theoretical 

 widths of the successive whorls being calculated by slide rule, the 

 corresponding actual specimens were then allocated to their places 

 and the diameters of the whorls necessarily taken from them. 

 The figures opposite the left of each whorl in the diagram 

 are the calculated measurements, the measurements and the 

 diameters of actual specimens being shown on the right. As was to 

 be expected, tfiese last do not always exhibit mathematical regularity, 

 as, for instance, in the case of the fifteenth to twentieth whorls, 

 but when the totals are cast up of the theoretical and actual 

 measurements they amount to 56-5 cm. and 56-2 cm. respectively, 

 a diiferejice of only 3 mm., showing that in nature what is lost in 

 one whorl is gained in the next, thus restoring the balance of the 

 whole. 



The sinistral spiral, although a complete apex of six whorls was 

 present, was less easy to determine owing to the lack of satisfactory 

 additional material. Taking the width of the sixth whorl, however, 

 at 6*5 cm. and calculating both ways from that, what material there 

 was pretty closely corresponded to a ratio of 1'055, and on these 

 lines a corresponding diagram was constructed. There is 

 nothing to show in this case what may have been the total length, 

 but we believe that the eleventh (with parts of the tenth and twelfth) 

 and the thirteenth to fifteenth whorls are represented. The portion 

 of another specimen, the last found, exhibits some five or six whorls, 

 of which the last two are imbedded in the concretionary mass to 

 which it was attached. Three of these, much distorted in 

 fossUization, were measurable and the results yielded were rather 

 remarkable, for taking the widths of each whorl at diametrically 

 opposite points (A and B) in centimetres the lollowitig table 

 originated : — 



Average Calculated width of 



A B width. whorls 14-16. 



8-5 11-7 . . . 10-1 . . 9-98 

 104 10-9 . . 10-6 . . 10-06 



9-4 11-7 . . . 10-5 . . 11-18 



Total . . 31-2 . . 31-22 



What the stroynbiformis of Schlotheim may have been cannot be guessed by 

 the description given. 



The later ascriptions to recent genera, which the fossil in no -wise resembles, 

 are to say the least unfortunate. Especially the last, for Pleurocera of 

 Rafinesque is a North American genus, whilst Sandberger's miscorrection of the 

 name to Pleiiroceras makes confusion with Hj-att's genus of Ammonites. 



Sowerby's trivial name may possibly prove valid for the British examples, 

 but a new generic name is desirable. 



