58 



MISS MATH) HEALEY ON 



[Feb. 1904, 



from being the case. For convenience of description, I have called 

 one surface A, the other B. Fig. 1 represents the B surface. On 

 surface A all the inner whorls are perfect : on B also they are 

 perfect, down to an umbilical diameter of 16 millimetres, that is 

 to the point in tig. 2. On side A of the cross-section there is 

 revealed a small, perfect ammonite, which can be traced to a 

 diameter of 46 millimetres, after which it is lost in a whorl the 

 sides of which are those of both the A and B surfaces, the shell 

 having here attained a diameter of 50 millimetres. The centres of 

 the two surfaces correspond, and so do the inner whorls. The 

 back of the little ammonite shows bifurcating ribs. It does not 

 seem possible that it could have been forced into its present posi- 

 tion by extraneous means, for neither the A nor the B surface shows 

 signs of disturbance in the inner whorls. On the other hand, it 

 is very difficult to imagine it growing in its present position. 



Remarks. — I have already indicated the probability of Peri- 

 tph'tactes biplex being a Kimeridgian form, but the horizon whence 

 it came must remain doubtful. In cases like this, it is perhaps 

 wisest to abandon the name altogether, or at least to restrict it 

 to the abnormal specimen to which it was first attached. 



I have not given dimensions, because no reliance can be placed 

 on them in such cases. 



The specimen is in the British Museum (Natural History). 



Pe risphixctes variocostatus (Buckland). (PI. XI, & text-fig. 2.) 

 [The 'type '-specimen.] 



1836. Ammonites variocostatus, Buckland, ' Bridsrewater Treatise' (no. 6) pi. xlii, 



fig. 7. 

 1898. Perisphinctes biplex, Siemiradzki, Pakeontographica, vol. xlv, ' Mono- 



graphische Besehreibung der Ammomtengattung Perisphinctes ' p. 265. 



Description. — The shell is large and discoidal. The whorls 



are about as high as 

 thick, and are orna- 

 mented with strong 

 ribs, directed slightly 

 forward and bifur- 

 cating with perfect 

 regularity as they 

 pass over on to the 

 back. At a diameter 

 of some 180 milli- 

 metres the whorls be- 

 come depressed, and 

 at the same time the 

 ribs become farther 

 apart, cease to bifur- 

 cate, and begin to develop wedge-shaped swellings. At a diameter 

 of 183 millimetres there are fifty-five ribs, and at 100 or thereabouts 

 (for the outer whorls prevent accurate measurements) fifty-three. 

 The suture-line delineated in tig. 2 (above) is restored. This was 



Fig. 2. — Suture-line of Perisphinctes vario- 

 costatus, restored. Nat. size. 



