494 E. T. Newton — On Ammoniies Jurenm. 



had not been found ; this view-, however, has been contested by 

 Mr. S. S. Buckman in a later number of the same journal. 



Mr. Jesson, who has been for some time collecting assiduously 

 from the Ironstone beds in the neighbourhood of Northampton, has 

 been kind enough to send to the Museum of Practical Geology, for 

 examination, the series of specimens which he has obtained. These 

 are chiefly Ammonites, some being from Duston, but the greater 

 number are from Brixworth, about seven miles N. of Northampton. 

 The species which have been determined are given in the list below, 

 but by far the greater number of specimens from the last-named 

 locality are referable to the two species Am. jurensis and Am. 

 opalinus, which are about equally numerous, while each of the 

 other species is represented by only two or three specimens. The 

 examples which are referred to Am,, jurensis are very uniform in 

 charactei", allowance being made for variation in size ; some of the 

 smaller (which may be the middle portions of larger ones) being an 

 inch or so in diameter, while others measure as much as six or eight 

 inches, and there are fragments of one specimen which must have 

 been at least 15 inches in diameter, or somewhat larger than the 

 example in the Museum of Practical Geology, figured half its 

 natural size by Dr. Wright,' with which these Brixworth Ammonites 

 exactly agree. 



The specimens which are referred to Am. opalinus correspond with 

 those figured by Mr. S. S. Buckman.'^ Am. insignis is represented 

 by two or three specimens in which the tubercles near the inner 

 n)argin are large, and in one example (4 or 5 inches in diameter) 

 almost obsolete. Another example has better defined tubercles, 

 until the Ammonite is about 5 or 6 inches in diameter, but the outer 

 whorl, which, if perfect, would be perhaps 10 inches in diameter, 

 is nearly smooth, having almost lost both ribs and tubercles. Two 

 or three examples of a variety of Am. insignis are included in this 

 collection, which agree with that figured by Dr. Wright^ as au 

 abnormal young specimen. 



The occurrence of Am. jurensis, insignis, opalinus, and Murchisona 

 in the Northampton Ironstone would seem to indicate that the 

 Ammonite zones are not so sharply defined in this area as they are 

 considered to be in some localities in the southern and western 

 counties; it is possible that future investigation may show the 

 ironstone series to be divisible into definite horizons, but at present 

 we have no evidence as to the particular part of the ironstone series 

 from which the fossils have been derived.* 



Fossils from the Ikonstone of Beixwoeth. 



Nautilus sp. 

 Belemnites sp. 

 Trigonia compta. 



,, V. scripta. 



Tancredia axiniformis ? 



opalinus. 



„ var. 

 Murchisonce. 



1 Pal. Soc, Lias Ammonites, pi. Ixxix. 



^ Pal. Soc. Inferior Oolite Ammonites, pi. xiii. 



3 Pal. Soc. Lias Ammonites, pi. Ixxv. figs 1-3. 



* This paper was read in Section (C) Geology, Brit. Assoc. Cardiff, Aug. 1891. 



