AEGOCERAS CATENATUM. 321 



I have quoted this remarkable passage from Sir Henry De la Beche's ' Manual,' as 

 it shows how the principles of palaeontology were understood in 1833 by one of the 

 greatest masters of geological science in the English or any other school. Had the true 

 value of Ammonites been known in his day, and the limits of the distribution of their 

 species in time and space been understood, the finding of a true Lias Ammonite would 

 have settled the question of the age of the limestone at La Spezia in which it was dis- 

 covered in spite of mistaking Belemnite phragmacones for Orthoceratites. 



The shell of Aecjoceras catenatum is compressed ; the sides are ornamented with 

 twenty-eight simple, sharp, very prominent ribs, arched upwards, backwards, and 

 forwards, becoming thicker as they approach the margin, and forming an arch over the 

 area, which is rather conspicuous in the figured specimen, whilst in others it is almost 

 smooth. The spine is formed of five convex whorls nearly one third involute, and all 

 seen in the umbilicus. The aperture is a compressed oval, cut out inwards by the turn 

 of the spire. The septa (fig. 7) are symmetrically divided on each side into three lobes 

 formed of unequal parts. The siphonal lobe is smaller than the principal lateral, and 

 terminates in three digitations ; the siphonal saddle is wide, and ends in three cells ; 

 the principal lateral lobe is long, with three digits on each side and a terminal seventh 

 having a trifid process ; the lateral saddle is as large as the principal lateral lobe, and 

 ends in two cells ; the lower lateral lobe is shorter than the principal, and has three 

 digits on one side, rudiments of digits on the other, and ends in three points ; the 

 first auxiliary lobe is oblique, with three inner digits, and the second auxiliary is 

 rudimentary. 



Acuities and Differences. — This species very much resembles some of the forms of 

 Aeg. angulatum; it differs, however, in the narrowness of the siphonal area, in retaining 

 its ribs entire up to an advanced age, and having a different lobe-line to that form. 



Locality and StratigrapMcal Position. — The specimens I possess of this Ammonite I 

 owe to the kindness of Mr. W. J. Harrison, F.G.S., late of the Leicester Museum ; they 

 were obtained from the Lower Lias beds at Barrow-on-Soar, and are largely charged 

 with iron -pyrites. I know of no other locality in the British Islands. 



Foreign Localities. — Professor d'Orbigny states that this Ammonite was found below 

 the Lower Lias beds with Grgphaea arcuata at Pont-Auber, near Avallon, Yonne ; Semur, 

 Cote-d'Or ; and that young specimens, figured by him in his plate 94 were collected on 

 the Gulf of La Spezia* near Coregna (Italy), thus determining the age of that terrain ; he 

 further adds that the so-called Orthoceratites of De la Beche, already referred to in my 

 extract as found in the same locality, as well as those from the Lake of Como, were the 

 articulations of the alveoli of Belevinites. This observation clears up the difficulty 

 which Sir Henry experienced in determining the age of the limestone at La Spezia. 



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