S. S. Biichnan — On Ainmonites Jurends. 259 



original Am. jurensis in the amount of involution, the extent of 

 compression, the very different-shaped umbilicus,^ and the broader 

 whorls. Even if the term * species ' be interpreted in a very liberal 

 sense, to include both, forms as jurensis, the differences remain the 

 same. Wright's fossil is not the typical jurensis ; and, moreover, it is 

 a form characteristic of the Opalinum-zone. Whether it be regarded 

 as a different species, or only a mutation of jurensis, it is advantageous 

 to mark its differences by a name — Lytoceras Wrighti. 



The evidence of the Jurensis-zone in the Northampton Sands is 

 thus narrowed down to Am. insignis. The typical form is highly 

 characteristic of the Jurensis-zone ; but there are several mutations 

 and allied species which are characteristic of the Opalinum-zone. 

 For instance, the "Insignis var.," which Newton says agrees with 

 Wright's pi. 75, figs. 1-3, is quite an aberrant form of Sammatoceras 

 (Insz^ms-group). It differs especially from insignis in ornamentation, 

 and in having ventral furrows. As it is not named, to my knowledge, 

 I think it might appropriately be called Hammatoceras Newtoui. 

 It occurs in Dorset quite high up in the Opalimim-zone. Its nearest 

 allied form is Am. Alleoni, Dumortier, of the same zone, which is 

 less coarsely ornate, less sulcate, and more involute. 



So far the bulk of Mr. Newton's evidence seems to indicate the 

 Northampton Sands being the Opalinum-zone, a conclusion I expressed 

 in Monogr. Amm. p. 53, 1888. The occurrence of Am. Murchisonce 

 in the Opalinum-zone has been noted by myself and several other 

 authors ; but the real head-quarters of Murchisonce are on a distinctly 

 higher level than the head-quarters of Opalinmn. 



As regards the Jurensis-zone in Northamptonshire, my position is 

 simply agnostic. I know no Ammonite-evidence, — barring, perhaps, 

 Mr. Newton's " insignis " — to show the existence of any of the four 

 levels — Dumortieria-, Dispanaum-, Striatulum-, Variabilis-heds — of this 

 zone, which are easily recognized by an abundant Ammonite-fauna 

 for about 80 miles from Haresfield to the Dorset coast ; but the 

 species of some or all may ultimately be found to occur in the 

 Northampton Sand, and I doubt not in the same successional order. 



Of the Opalinum-zone there is good evidence for the existence of 

 the upper level Opali7ium-heds ; of the lower level, Moorei-heds, 

 Lytoceras Wrighti appears to be the only evidence. This is not 

 sufficient — see sections Monogr. Amm. p. 45, et seq. 



A summary of the species allied to or similar to A. jurensis may 

 be useful in this connexion. As their genealogy, synonymy, etc., 

 will be given more fully in a future paper, only the briefest notice 

 will be made. 



1. Am. jurensis, Zieten. Oval whorls, very little involute. Very 

 rare. Burton Bradstock, Yeovil Sands. 



2. Am. perlcevis, Denckmann (usually called jurensis). It differs 

 only in being more compressed ventrally, and having a more pro- 

 nounced inner margin. Scarce. Dispansum-heds, Cotteswolds. 



3. Am. phyllicinctus, Quenstedt. Like the last, but more evolute. 



^ The iimbilicus of the typical Wrighti is basia-shaped, that of the ij'gical jurensis 

 ■wider and graduated. 



